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/schemas.ts

@@ -16,6 +16,7 @@ import {
   MIN_TERMINAL_BUFFER_BYTES,
   MIN_TERMINAL_SCROLLBACK_LINES,
 } from '../config/terminal-history.js';
+import { MAX_EDITABLE_BYTES } from '../config/file-editing.js';
 
 // ========== Path Validation ==========
 
@@ -83,6 +84,30 @@ export const FilesystemPreviewQuerySchema = z.object({
     .optional(),
 });
 
+/**
+ * Body validation for `PUT /api/sessions/:id/file-content` (File Viewer edit
+ * mode). `content.max()` counts UTF-16 code units, which for UTF-8 output is
+ * always <= the byte length, so it is a coarse pre-filter that never rejects
+ * valid content; the handler enforces the exact MAX_EDITABLE_BYTES byte cap.
+ * Workspace containment and symlink resolution are enforced by the route via
+ * validateSessionFilePath after parsing.
+ */
+export const FileWriteSchema = z
+  .object({
+    path: z
+      .string()
+      .min(1)
+      .max(4096)
+      .refine((p) => !p.includes('\0') && !p.includes('\n') && !p.includes('\r'), {
+        message: 'Invalid path',
+      }),
+    content: z.string().max(MAX_EDITABLE_BYTES),
+    baseHash: z.string().regex(/^[a-f0-9]{64}$/, 'baseHash must be a sha256 hex digest'),
+    eol: z.enum(['lf', 'crlf']).optional(),
+    force: z.boolean().optional(),
+  })
+  .strict();
+
 // ========== Env Var Allowlist ==========
 
 /** Allowlisted env var key prefixes */