fix: session-cwd attach race, terminal failure surfacing, white-screen hardening; CodeMirror editor

- cwd resolution falls back (attached header → client summary cwd → process
  cwd) across the JSON API, media route and terminal upgrade, so explorer/
  git/terminal work from first paint instead of 'session ... has no working
  directory' while the web client hydrates the session (refresh-only fix).
- terminal client threads the summary cwd into the WS URL and stops the
  reconnect loop after three unreasoned failures, showing the close code
  with a manual retry; a 1011+reason refusal already showed the reason.
- white screen: persisted state is structurally sanitized on load (malformed
  shapes fall back to default) and the restored width is clamped to the
  viewport; the layout margin is capped at the viewport; the client entry
  wraps every lifecycle step in try/catch and pins a visible diagnostic bar
  on load/mount/interception failure instead of failing silently.
- editor: textarea replaced with CodeMirror 6 — line wrapping, per-extension
  syntax highlighting (js/ts/jsx/tsx, json, python, html, css, xml, yaml,
  sql, java, c/c++, rust, go, php, shell, toml, nginx, dockerfile,
  properties), one-dark theme, dirty dot + Ctrl/Cmd+S save kept.
- tests: +9 (cwd fallback chain over the API route, state sanitization,
  language mapping); 36 pass, typecheck + build + vm materialize clean.

src/client/EditorView.tsx

@@ -1,46 +1,48 @@
 /**
- * The file editor tab: text editing with a dirty dot and Ctrl/Cmd+S save,
+ * The file editor tab: a CodeMirror 6 editor with line wrapping and syntax
+ * highlighting (extension-keyed language), a dirty dot and Ctrl/Cmd+S save,
  * image viewing through the media route, Markdown preview through the shared
  * MarkdownText component, and a plain notice for binary files. Reads cap at
  * the host's 512KB bound (a banner marks truncation).
  */
-import { useEffect, useState } from 'react'
+import { useEffect, useRef, useState } from 'react'
 import clsx from 'clsx'
+import { EditorState } from '@codemirror/state'
+import { EditorView as CodeMirrorView, keymap } from '@codemirror/view'
+import { defaultKeymap, history, historyKeymap } from '@codemirror/commands'
+import { oneDark } from '@codemirror/theme-one-dark'
 import { IconCheckOutline16, MarkdownText } from '@deepseek-ai/dsh-client-ui-primitives'
-import { api, mediaUrl } from './api.ts'
+import { api, mediaUrl, type SessionScope } from './api.ts'
+import { languageForPath } from './lang.ts'
 import { t } from './locales.ts'
 import css from './sidebar.module.css'
 
 const IMAGE_EXT = ['.png', '.jpg', '.jpeg', '.gif', '.webp', '.svg', '.bmp', '.ico', '.avif']
 const MD_EXT = ['.md', '.markdown']
 
-function extOf(path: string): string {
-  const at = path.lastIndexOf('.')
-  if (at === -1) return ''
-  const base = path.slice(at).toLowerCase()
-  return base.includes('/') || base.includes('\\') ? '' : base
-}
-
 type EditorLoad =
   | { status: 'loading' }
   | { status: 'error'; message: string }
   | { status: 'ready'; kind: 'text' | 'image' | 'md'; content: string; truncated: boolean }
   | { status: 'binary' }
 
-export function EditorView(props: { sessionId: string; path: string; title: string }) {
-  const { sessionId, path, title } = props
+export function EditorView(props: { scope: SessionScope; path: string; title: string }) {
+  const { scope, path, title } = props
   const [load, setLoad] = useState<EditorLoad>({ status: 'loading' })
-  const [draft, setDraft] = useState<string | null>(null)
+  const [dirty, setDirty] = useState(false)
   const [saveState, setSaveState] = useState<'idle' | 'saving' | 'saved' | 'failed'>('idle')
+  const hostRef = useRef<HTMLDivElement>(null)
+  const viewRef = useRef<CodeMirrorView | null>(null)
+  const savingRef = useRef(false)
 
   useEffect(() => {
     let cancelled = false
     setLoad({ status: 'loading' })
-    setDraft(null)
+    setDirty(false)
     setSaveState('idle')
-    api.fsRead(sessionId, path).then((result) => {
+    api.fsRead(scope, path).then((result) => {
       if (cancelled) return
-      const ext = extOf(path)
+      const ext = extOfPath(path)
       if (result.kind === 'binary') {
         setLoad({ status: 'binary' })
         return
@@ -60,20 +62,66 @@ export function EditorView(props: { sessionId: string; path: string; title: stri
       setLoad({ status: 'error', message: error instanceof Error ? error.message : String(error) })
     })
     return () => { cancelled = true }
-  }, [sessionId, path])
+  }, [scope.sessionId, scope.cwd, path])
+
+  // Create the CodeMirror editor once the text is loaded. The view owns the
+  // document; React only tracks dirty/save state through the update listener.
+  useEffect(() => {
+    if (load.status !== 'ready' || load.kind !== 'text') return
+    const host = hostRef.current
+    if (host === null) return
+    const language = languageForPath(path)
+    const state = EditorState.create({
+      doc: load.content,
+      extensions: [
+        CodeMirrorView.lineWrapping,
+        history(),
+        EditorState.tabSize.of(2),
+        CodeMirrorView.contentAttributes.of({ spellcheck: 'false' }),
+        CodeMirrorView.theme({
+          '&': { height: '100%', fontSize: '13px', backgroundColor: 'transparent' },
+          '.cm-scroller': { overflow: 'auto', fontFamily: '"SF Mono", Menlo, Consolas, "Liberation Mono", monospace' },
+          '.cm-content': { caretColor: 'var(--dsw-alias-label-primary)' },
+        }),
+        ...(language !== null ? [language] : []),
+        oneDark,
+        CodeMirrorView.updateListener.of((update) => {
+          if (update.docChanged) setDirty(true)
+        }),
+        keymap.of([
+          {
+            key: 'Mod-s',
+            preventDefault: true,
+            run: () => { save(); return true },
+          },
+          ...defaultKeymap,
+          ...historyKeymap,
+        ]),
+      ],
+    })
+    const view = new CodeMirrorView({ state, parent: host })
+    viewRef.current = view
+    return () => {
+      view.destroy()
+      viewRef.current = null
+    }
+  }, [load])
 
   const save = (): void => {
-    if (load.status !== 'ready' || load.kind !== 'text' || draft === null || saveState === 'saving') return
+    const view = viewRef.current
+    if (view === null || savingRef.current) return
+    savingRef.current = true
     setSaveState('saving')
-    api.fsWrite(sessionId, path, draft).then(() => {
+    api.fsWrite(scope, path, view.state.doc.toString()).then(() => {
+      savingRef.current = false
+      setDirty(false)
       setSaveState('saved')
-      setDraft(null)
     }).catch(() => {
+      savingRef.current = false
       setSaveState('failed')
     })
   }
 
-  const dirty = load.status === 'ready' && load.kind === 'text' && draft !== null
   const saveLabel = saveState === 'saving' ? t('loading') : saveState === 'saved' ? t('saved') : saveState === 'failed' ? t('saveFailed') : ''
 
   return (
@@ -100,26 +148,12 @@ export function EditorView(props: { sessionId: string; path: string; title: stri
       {load.status === 'ready' && load.kind === 'text' && (
         <>
           {load.truncated && <div className={css.editorBanner}>{t('truncation')}</div>}
-          <textarea
-            className={css.editorTextarea}
-            spellCheck={false}
-            value={draft ?? load.content}
-            onChange={(event) => {
-              setDraft(event.target.value)
-              setSaveState('idle')
-            }}
-            onKeyDown={(event) => {
-              if ((event.ctrlKey || event.metaKey) && event.key === 's') {
-                event.preventDefault()
-                save()
-              }
-            }}
-          />
+          <div className={css.editorCm} ref={hostRef} />
         </>
       )}
       {load.status === 'ready' && load.kind === 'image' && (
         <div className={css.editorImageWrap}>
-          <img className={css.editorImage} src={mediaUrl(sessionId, path)} alt={title} />
+          <img className={css.editorImage} src={mediaUrl(scope, path)} alt={title} />
         </div>
       )}
       {load.status === 'ready' && load.kind === 'md' && (
@@ -130,3 +164,11 @@ export function EditorView(props: { sessionId: string; path: string; title: stri
     </div>
   )
 }
+
+/** The lowercased file extension of a path ('' when none). */
+function extOfPath(path: string): string {
+  const at = path.lastIndexOf('.')
+  if (at === -1) return ''
+  const base = path.slice(at).toLowerCase()
+  return base.includes('/') || base.includes('\\') ? '' : base
+}