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/locales.ts

@@ -21,6 +21,7 @@ const zh = {
   expandFullscreen: '展开为全屏',
   restoreFullscreen: '还原宽度',
   terminalError: '终端连接失败',
+  terminalConnectFailed: '终端多次连接失败',
   terminalRetry: '重试',
   refresh: '刷新',
   save: '保存',
@@ -82,6 +83,7 @@ const en: Record<keyof typeof zh, string> = {
   expandFullscreen: 'Expand to full width',
   restoreFullscreen: 'Restore width',
   terminalError: 'Terminal connection failed',
+  terminalConnectFailed: 'Terminal failed to connect repeatedly',
   terminalRetry: 'Retry',
   refresh: 'Refresh',
   save: 'Save',