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/styles.css

@@ -9430,6 +9430,84 @@ kbd {
   flex-shrink: 0;
 }
 
+/* ---- File Viewer edit mode (issue #212) ---- */
+
+.file-preview-body textarea.file-preview-editor {
+  display: block;
+  width: 100%;
+  height: 100%;
+  margin: 0;
+  padding: 0.75rem;
+  border: none;
+  outline: none;
+  resize: none;
+  background: var(--bg-dark);
+  color: var(--text);
+  font-family: var(--font-mono);
+  font-size: 0.8rem;
+  line-height: 1.5;
+  white-space: pre;
+  overflow-wrap: normal;
+  overflow: auto;
+  tab-size: 4;
+}
+
+.file-preview-editbar {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  padding: 0.4rem 0.75rem;
+  border-top: 1px solid var(--border);
+  background: var(--bg-input);
+  flex-shrink: 0;
+}
+
+.file-preview-editbar[hidden] {
+  display: none;
+}
+
+.file-preview-editbar-spacer {
+  flex: 1;
+}
+
+.file-preview-dirty {
+  font-size: 0.7rem;
+  color: var(--warning, #e5c07b);
+}
+
+.file-preview-dirty::before {
+  content: '\25CF  ';
+}
+
+.file-preview-editbar-btn {
+  padding: 0.3rem 0.9rem;
+  font-size: 0.75rem;
+  border-radius: 6px;
+  border: 1px solid var(--control-border);
+  background: var(--bg-input);
+  color: var(--text);
+  cursor: pointer;
+}
+
+.file-preview-editbar-btn:hover {
+  background: var(--bg-hover, rgba(255, 255, 255, 0.08));
+}
+
+.file-preview-editbar-btn--save {
+  background: var(--accent);
+  border-color: var(--accent);
+  color: #fff;
+}
+
+.file-preview-editbar-btn--save:hover {
+  background: var(--accent-hover);
+}
+
+.file-preview-editbar-btn--save:disabled {
+  opacity: 0.45;
+  cursor: default;
+}
+
 /* ========== Log Viewer Windows (Floating) ========== */
 
 .log-viewer-window {