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/mobile.css

@@ -1871,6 +1871,33 @@ html.mobile-init .file-browser-panel {
     bottom: calc(44px + 2rem + var(--safe-area-bottom));
   }
 
+  /* File preview window: full screen on phones. --app-height tracks the visual
+     viewport (KeyboardHandler), so the edit textarea + Save bar stay above the
+     OS keyboard instead of hiding behind it. Footer/edit bar pad for the home
+     indicator. */
+  .file-preview-window {
+    width: 100vw;
+    max-width: 100vw;
+    height: var(--app-height, 100vh);
+    max-height: var(--app-height, 100vh);
+    border-radius: 0;
+    border-left: none;
+    border-right: none;
+  }
+
+  .file-preview-editbar {
+    padding-bottom: calc(0.4rem + var(--safe-area-bottom));
+  }
+
+  .file-preview-overlay .file-preview-footer {
+    padding-bottom: calc(0.35rem + var(--safe-area-bottom));
+  }
+
+  /* >=16px or iOS Safari auto-zooms the page on focus */
+  .file-preview-body textarea.file-preview-editor {
+    font-size: 16px;
+  }
+
   /* Notification drawer - full width on mobile, includes safe area padding */
   .notification-drawer {
     width: 100%;