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/TreePanel.tsx

@@ -19,6 +19,7 @@ import { useEffect, useRef, useState, type InputHTMLAttributes } from 'react'
 import clsx from 'clsx'
 import { IconFolderOpen16, IconRefreshOutline16 } from '@deepseek-ai/dsh-client-ui-primitives'
 import { api } from './api.ts'
+import type { SidebarStore } from './state.ts'
 import { FileTree } from './FileTree.tsx'
 import { IconUploadOutline16 } from './icons.tsx'
 import type { OpenWithTarget } from './open-with.ts'
@@ -44,6 +45,8 @@ interface UploadSession {
 export function TreePanel(props: {
   sessionId: string
   cwd: string | undefined
+  /** The sidebar store (passed through to the tree's fence-refusal notice). */
+  store: SidebarStore
   expanded: string[]
   revealed: string[]
   onToggle: (path: string) => void
@@ -64,7 +67,7 @@ export function TreePanel(props: {
    *  at a fixed width. */
   full?: boolean
 }) {
-  const { sessionId, cwd, expanded, revealed, onToggle, onOpenFile, onOpenFileNewTab, onOpenFileSide, openWithTargets, openWithPinned, openWithSsh, onOpenWith, onToggleOpenWithPin, onReferenceFile, full } = props
+  const { sessionId, cwd, store, expanded, revealed, onToggle, onOpenFile, onOpenFileNewTab, onOpenFileSide, openWithTargets, openWithPinned, openWithSsh, onOpenWith, onToggleOpenWithPin, onReferenceFile, full } = props
   const [query, setQuery] = useState('')
   const [results, setResults] = useState<{ matches: string[]; truncated: boolean } | null>(null)
   const [error, setError] = useState<string | null>(null)
@@ -221,6 +224,7 @@ export function TreePanel(props: {
         <FileTree
           sessionId={sessionId}
           cwd={cwd}
+          store={store}
           expanded={expanded}
           revealed={revealed}
           onToggle={onToggle}