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/config/file-editing.ts

@@ -0,0 +1,152 @@
+/**
+ * @fileoverview File Viewer edit-mode policy (issue #212).
+ *
+ * Pure, IO-free policy for which workspace files the in-viewer editor may read
+ * for editing and write back. Consumed by the `edit=1` branch of
+ * `GET /api/sessions/:id/file-content` and by `PUT /api/sessions/:id/file-content`
+ * in `src/web/routes/file-routes.ts`.
+ *
+ * Design (docs/file-viewer-edit-plan.md):
+ * - ALLOWLIST of text extensions/basenames, not a blocklist — matching the
+ *   attachment-guard precedent. `svg` and `env` are deliberately absent: svg is
+ *   treated as untrusted on the read side, and `.env` is sensitive-path blocked
+ *   anyway; excluding them here keeps a single obvious refusal.
+ * - The `.git/` subtree is denied outright: `.git/hooks/*` is code execution and
+ *   a corrupted index looks unrecoverable to a user who wanted to fix a typo.
+ * - EOL helpers exist because a browser <textarea> normalizes to LF; the server
+ *   re-applies the file's original ending so a two-line edit of a CRLF file does
+ *   not become a whole-file diff. Mixed-EOL files normalize to the dominant
+ *   style (documented lossy edge).
+ */
+
+/** Hard cap for edit-mode reads AND writes (bytes of file content). */
+export const MAX_EDITABLE_BYTES = 512 * 1024;
+
+/** Lowercase extensions (no dot) the editor will open and save. */
+export const EDITABLE_EXTENSIONS: ReadonlySet<string> = new Set([
+  // JS/TS ecosystem
+  'ts',
+  'tsx',
+  'js',
+  'jsx',
+  'mjs',
+  'cjs',
+  'json',
+  'jsonc',
+  // Docs / plain text
+  'md',
+  'mdx',
+  'txt',
+  'rst',
+  'adoc',
+  // Web
+  'css',
+  'scss',
+  'less',
+  'html',
+  'htm',
+  'xml',
+  // Config
+  'yml',
+  'yaml',
+  'toml',
+  'ini',
+  'cfg',
+  'conf',
+  'properties',
+  // Shell
+  'sh',
+  'bash',
+  'zsh',
+  'fish',
+  // Languages
+  'py',
+  'rb',
+  'go',
+  'rs',
+  'java',
+  'kt',
+  'swift',
+  'c',
+  'h',
+  'cpp',
+  'hpp',
+  'cc',
+  'cs',
+  'php',
+  'sql',
+  'graphql',
+  'proto',
+  'lua',
+  'pl',
+  'r',
+  'jl',
+  'tf',
+  'gradle',
+  // Data / misc text
+  'csv',
+  'tsv',
+  'log',
+  'diff',
+  'patch',
+]);
+
+/** Extensionless (or dot-led) file names that are still editable text. */
+export const EDITABLE_BASENAMES: ReadonlySet<string> = new Set([
+  'dockerfile',
+  'makefile',
+  'license',
+  'readme',
+  'changelog',
+  'authors',
+  'codeowners',
+  'procfile',
+  '.gitignore',
+  '.gitattributes',
+  '.dockerignore',
+  '.prettierignore',
+  '.prettierrc',
+  '.editorconfig',
+  '.nvmrc',
+  '.npmrc',
+  '.eslintignore',
+]);
+
+/** Whether a file name (basename only) is eligible for in-viewer editing. */
+export function isEditableFileName(fileName: string): boolean {
+  const lower = fileName.toLowerCase();
+  if (EDITABLE_BASENAMES.has(lower)) return true;
+  const dot = lower.lastIndexOf('.');
+  // No extension (or a bare dotfile like `.bashrc`): only the basename list applies.
+  if (dot <= 0) return false;
+  return EDITABLE_EXTENSIONS.has(lower.slice(dot + 1));
+}
+
+/**
+ * Whether a workspace-relative path is denied for editing regardless of its
+ * extension. Currently: anything inside a `.git` directory at any depth.
+ */
+export function isDeniedEditRelativePath(relativePath: string): boolean {
+  return relativePath.split('/').some((segment) => segment === '.git');
+}
+
+export type FileEol = 'lf' | 'crlf';
+
+/** Dominant line-ending style of a text buffer (LF when tied or single-line). */
+export function detectEol(text: string): FileEol {
+  let crlf = 0;
+  let lf = 0;
+  for (let i = 0; i < text.length; i++) {
+    if (text.charCodeAt(i) === 10) {
+      if (i > 0 && text.charCodeAt(i - 1) === 13) crlf++;
+      else lf++;
+    }
+  }
+  return crlf > lf ? 'crlf' : 'lf';
+}
+
+/** Normalize every line ending in `text` to the requested style. */
+export function applyEol(text: string, eol: FileEol): string {
+  const normalized = text.replace(/\r\n/g, '\n');
+  return eol === 'crlf' ? normalized.replace(/\n/g, '\r\n') : normalized;
+}