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.

tests/smoke.spec.ts

@@ -16,7 +16,7 @@ interface FakeContext {
     register: (route: SidebarWebRoute) => () => void
     registerUpgrade: (route: SidebarWebUpgradeRoute) => () => void
   }
-  sessions: { get: (id: string) => undefined }
+  sessions: { get: (id: string) => { header: { cwd?: string } } | undefined }
   effect: (fn: () => void | (() => void), label?: string) => void
 }
 
@@ -126,3 +126,74 @@ describe('host plugin smoke', () => {
     expect(listing.truncated).toBe(false)
   })
 })
+
+describe('session cwd resolution over the API route', () => {
+  interface CtxOverrides {
+    sessions?: { get: (id: string) => { header: { cwd?: string } } | undefined }
+  }
+
+  const mount = (overrides: CtxOverrides = {}): SidebarWebRoute => {
+    const routes: SidebarWebRoute[] = []
+    const ctx = {
+      loader: { entries: () => [] },
+      httpServer: {
+        register: (route: SidebarWebRoute) => { routes.push(route); return () => {} },
+        registerUpgrade: (route: SidebarWebUpgradeRoute) => { void route; return () => {} },
+      },
+      sessions: overrides.sessions ?? { get: () => undefined },
+      // The vendored cordis runs registration effects immediately.
+      effect: (fn: () => void | (() => void)) => { fn() },
+    }
+    apply(ctx as never)
+    return routes.find(route => route.path === '/sidebar/api')!
+  }
+
+  const invoke = async (route: SidebarWebRoute, payload: unknown): Promise<{ ok: boolean; value?: { cwd: string }; error?: { message: string } }> => {
+    const body = Buffer.from(JSON.stringify(payload))
+    const req = {
+      method: 'POST',
+      url: '/sidebar/api/session.cwd',
+      headers: { host: '127.0.0.1:3080' },
+      [Symbol.asyncIterator]: async function* () { yield body },
+    } as never
+    const out: { status: number; body: string } = { status: 200, body: '' }
+    const res = {
+      writeHead: (status: number) => { out.status = status },
+      end: (chunk: unknown) => { out.body += String(chunk ?? '') },
+    } as never
+    await route.handler(req, res)
+    return JSON.parse(out.body) as { ok: boolean; value?: { cwd: string }; error?: { message: string } }
+  }
+
+  it('uses the client summary cwd while the session is detached', async () => {
+    const route = mount()
+    const result = await invoke(route, { sessionId: 's-detached', cwd: '/tmp/summary-cwd' })
+    expect(result.ok).toBe(true)
+    expect(result.value?.cwd).toBe('/tmp/summary-cwd')
+  })
+
+  it('falls back to the process cwd with no summary cwd', async () => {
+    const route = mount()
+    const result = await invoke(route, { sessionId: 's-unknown' })
+    expect(result.ok).toBe(true)
+    expect(result.value?.cwd).toBe(process.cwd())
+  })
+
+  it('prefers the attached session header over the client summary', async () => {
+    const route = mount({
+      sessions: {
+        get: (id) => id === 's-attached' ? { header: { cwd: '/attached-cwd' } } : undefined,
+      },
+    })
+    const result = await invoke(route, { sessionId: 's-attached', cwd: '/tmp/summary-cwd' })
+    expect(result.ok).toBe(true)
+    expect(result.value?.cwd).toBe('/attached-cwd')
+  })
+
+  it('rejects a non-absolute client cwd', async () => {
+    const route = mount()
+    const result = await invoke(route, { sessionId: 's-detached', cwd: 'relative/path' })
+    expect(result.ok).toBe(false)
+    expect(result.error?.message).toMatch(/invalid working directory/)
+  })
+})

tests/unit.spec.ts

@@ -3,8 +3,9 @@ import { compareEntries, parentOf, rootLabel, requireAbsolute } from '../src/fs-
 import { parseLogLines, parsePorcelainZ } from '../src/git.ts'
 import {
   activateTab, closeTab, makeDefaultState, moveTab, moveTabToEdge, openTab,
-  resizeSplit, splitPane, toggleExpanded, type SidebarState, type SplitNode,
+  resizeSplit, sanitizeState, splitPane, toggleExpanded, type SidebarState, type SplitNode,
 } from '../src/client/state.ts'
+import { extOf, languageKeyForExt } from '../src/client/lang.ts'
 import { producedForClosing, resolveSidebarPath, selectProducedFiles } from '../src/client/produced-files.ts'
 import { defaultShell, ensureSpawnHelper } from '../src/pty-manager.ts'
 
@@ -260,3 +261,57 @@ describe('produced-files derivation', () => {
     expect(resolveSidebarPath(undefined, 'a.ts')).toBe('a.ts')
   })
 })
+
+describe('persisted state sanitization', () => {
+  it('accepts a well-formed state unchanged (node environment: no width clamp)', () => {
+    const state = makeDefaultState(400)
+    const clean = sanitizeState(JSON.parse(JSON.stringify(state)))
+    expect(clean).toEqual(state)
+  })
+
+  it('clamps undersized widths to the panel minimum', () => {
+    const state = { ...makeDefaultState(400), width: 10 }
+    const clean = sanitizeState(JSON.parse(JSON.stringify(state)))
+    expect(clean?.width).toBe(280)
+  })
+
+  it('rejects malformed shapes instead of crashing the panel', () => {
+    expect(sanitizeState(null)).toBeUndefined()
+    expect(sanitizeState('nope')).toBeUndefined()
+    expect(sanitizeState({})).toBeUndefined()
+    expect(sanitizeState({ ...makeDefaultState(400), width: 'wide' })).toBeUndefined()
+    expect(sanitizeState({ ...makeDefaultState(400), panelOpen: 1 })).toBeUndefined()
+    // A split whose sizes do not match its children is rejected.
+    const withSplit = JSON.parse(JSON.stringify(makeDefaultState(400)))
+    withSplit.splits = { kind: 'split', id: 's1', dir: 'row', sizes: [0.5], children: [] }
+    expect(sanitizeState(withSplit)).toBeUndefined()
+    // Unknown tab types (older layouts) are rejected.
+    const withBadTab = JSON.parse(JSON.stringify(makeDefaultState(400)))
+    withBadTab.splits.tabs[0].type = 'watcher'
+    expect(sanitizeState(withBadTab)).toBeUndefined()
+    // An active id that no tab carries is rejected.
+    const withBadActive = JSON.parse(JSON.stringify(makeDefaultState(400)))
+    withBadActive.splits.active = 'ghost-tab'
+    expect(sanitizeState(withBadActive)).toBeUndefined()
+  })
+})
+
+describe('editor language mapping', () => {
+  it('derives extensions from paths', () => {
+    expect(extOf('/a/b/main.tsx')).toBe('tsx')
+    expect(extOf('README.MD')).toBe('md')
+    expect(extOf('/a/b/.gitignore')).toBe('gitignore')
+    expect(extOf('noext')).toBe('')
+  })
+
+  it('maps common extensions to languages and falls back to plain text', () => {
+    expect(languageKeyForExt('tsx')).toBe('tsx')
+    expect(languageKeyForExt('js')).toBe('js')
+    expect(languageKeyForExt('py')).toBe('python')
+    expect(languageKeyForExt('yaml')).toBe('yaml')
+    expect(languageKeyForExt('sh')).toBe('shell')
+    expect(languageKeyForExt('txt')).toBeNull()
+    expect(languageKeyForExt('log')).toBeNull()
+    expect(languageKeyForExt('')).toBeNull()
+  })
+})