feat: workspace fence toggle (workspaceFence pref + one-click off in error surfaces)

- New persisted built-in pref workspaceFence (default true) under the
  files card's secondary settings modal; host reads it per request via
  the settings seam (fenceEnabledOf — absent/malformed stays ON).
- ensureWorkspacePath/ensureWorkspaceWritePath gain a fence param:
  disarmed skips the containment assert but keeps realpath resolution;
  wired through fs.tree/fs.read/fs.write, the media + html routes, and
  the upload path (writeWorkspaceUpload fence input).
- Editor load failures and file-tree level errors show the localized
  reason plus a one-click global off (FenceErrorNotice) that flips the
  pref, adopts it into the store, and auto-retries the failed operation
  (reloadSeq / retryDir) — the raw wire text is never shown.
- GitView's two client-side pre-checks (hidden open action + no-op
  defense) respect the pref; store threaded through TreePanel/FileTree.
- i18n: 4 new keys across all 21 dictionaries; tests: fence-off route
  coverage, prefs parsing, schema default, editor toggles list;
  docs: guide §8/§8.1 + plans/2026-08-29 design record.

src/client/prefs.ts

@@ -82,6 +82,9 @@ export function parsePrefs(value: unknown): SidebarPrefs {
     editorExplorer: typeof record.editorExplorer === 'boolean'
       ? record.editorExplorer
       : SIDEBAR_PREFS_DEFAULTS.editorExplorer,
+    workspaceFence: typeof record.workspaceFence === 'boolean'
+      ? record.workspaceFence
+      : SIDEBAR_PREFS_DEFAULTS.workspaceFence,
     // The title-bar scheme (auto | web | preset | custom). The schema
     // declares the field WITHOUT a default, so documents written by older
     // plugin versions resolve without it — migrate from the legacy fields: