feat(file-viewer): edit mode for text files (edit + save in the viewer)

Closes #212. The file-preview overlay can now edit workspace text files in
place, phone-first: agent writes a file, you review it in the viewer, tweak
two lines, save, tell the agent to continue.

Backend (file-routes.ts, policy in src/config/file-editing.ts):
- GET file-content?edit=1: read-for-edit that never truncates (a truncated
  buffer must never become an edit buffer), 512KB cap (413 over it), and
  returns the sha256 hash + detected EOL the client echoes back on save.
- PUT /api/sessions/:id/file-content: edit-in-place only, with no O_CREAT
  anywhere in the handler. Confinement matches the read path (realpath +
  workspace boundary + ownership via findSessionOrFail), plus sensitive-path
  and attachment-guard blocklists, a .git subtree deny, and an extension
  allowlist (svg and env deliberately excluded). Optimistic concurrency via
  baseHash: mismatch is a 409 unless force. Writes are wx-temp + fchmod +
  fsync + rename, closing the validate-then-write TOCTOU window.
- Corruption guards: NUL sniff + UTF-8 round-trip compare (refuses binary
  and latin-1), and server-side EOL re-application so a textarea's LF
  normalization cannot rewrite every line of a CRLF file.
- Plain reads gain an additive editable flag the UI keys the button off.

Frontend (panels-ui.js + overlay markup/styles):
- Edit button on editable text previews; textarea editor with Save/Cancel,
  dirty indicator, discard-confirm on cancel/close, and a conflict dialog
  that offers overwrite (force) when the file changed on disk mid-edit.
- Phone: full-bleed window sized by --app-height so the editor and Save bar
  track the OS keyboard; 16px editor font (iOS zoom guard); no autofocus.
- zh-CN strings for the new chrome.

Tests: pure policy unit tests plus a route suite that deliberately does NOT
mock node:fs. It runs against a real temp workspace so symlink escapes,
write-through of in-workspace symlinks, mode preservation, CRLF round-trip,
409/force, and the no-create property are exercised for real. Also verified
end to end on an isolated beta instance: 39-check curl matrix, Playwright
desktop flow (real clicks and typing, bytes asserted on disk, live conflict
with an external rewrite), and a 393px phone profile.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/web/public/i18n.js

@@ -600,6 +600,9 @@
     'Select a run to view its agents': '选择一次运行以查看其智能体',
     'Source type filter': '来源类型筛选',
     'Copy content': '复制内容',
+    'Edit file': '编辑文件',
+    'Unsaved changes': '未保存的更改',
+    Saved: '已保存',
     'Export as JSON': '导出为 JSON',
     'Export as Markdown': '导出为 Markdown',
     'Mark all read': '全部标为已读',