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/sidebar.module.css

@@ -1493,6 +1493,18 @@ body[data-dsh-sidebar-dragging] .bottomPanel {
   color: var(--dsw-alias-state-error-primary);
 }
 
+/* The workspace-fence refusal row (FenceErrorNotice): reason text plus the
+ * one-click global-off button share one baseline, wrapping when narrow. */
+.fenceError {
+  display: flex;
+  align-items: center;
+  flex-wrap: wrap;
+  gap: 8px;
+  padding: 8px 16px;
+  font: var(--dsw-font-xxs-12);
+  color: var(--dsw-alias-state-error-primary);
+}
+
 .editorBanner {
   flex: none;
   padding: 4px 8px;