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/sidebar.module.css

@@ -538,19 +538,21 @@
   background: var(--dsw-alias-state-warn-tertiary);
 }
 
-.editorTextarea {
+.editorCm {
   flex: 1;
   min-height: 0;
-  width: 100%;
-  padding: 10px 12px;
-  border: none;
-  outline: none;
-  resize: none;
+  overflow: hidden;
   background: transparent;
-  color: var(--dsw-alias-label-primary);
-  font: var(--dsw-font-markdown-code-block-small);
-  tab-size: 2;
-  white-space: pre;
+}
+
+/* CodeMirror fills the pane; the theme supplies its own token palette
+   (a designed dark theme, same policy as the xterm palette). */
+.editorCm :global(.cm-editor) {
+  height: 100%;
+}
+
+.editorCm :global(.cm-editor.cm-focused) {
+  outline: none;
 }
 
 .editorImageWrap {