feat(cockpit): approval preview — command/diff above Approve/Reject (W1)

Remote one-tap approval was blind (you'd tap Approve without seeing Claude wants
to run `rm -rf` or rewrite a config). The pending tool's actual command / diff now
renders above the approval bar, on every attached device, riding the same broadcast
+ late-joiner rails as the existing `gate` field.

- src/http/approval-preview.ts (new, pure, never-throws): deriveApprovalPreview —
  Bash → command; Edit/Write/MultiEdit/NotebookEdit → a synthetic DiffFile; else null.
  Every line sanitized via sanitizeField (strips control/ANSI); caps 40 lines /
  200 chars/line / 4KB (security limits, UTF-8-safe byte clamp).
- src/types.ts: additive optional `preview?: ApprovalPreview` on the status
  ServerMessage + handleHookEvent (older clients ignore it).
- src/server.ts /hook/permission: derive preview from tool_input, store on the
  PendingApproval entry, re-send to late joiners exactly like `gate`.
- manager.ts threads it; public/tabs.ts renderApprovalPreview (command → <pre>
  textContent; diff → reused innerHTML-free renderDiffFile). Unknown tools /
  plan gates fall back to today's name-only bar.

Attacker-influenced tool input → rendered via textContent/diff-renderer only,
never innerHTML. Verified independently: typecheck + build:web clean, 1692 pass.

src/session/manager.ts

@@ -26,6 +26,7 @@
  */
 
 import type {
+  ApprovalPreview,
   ClaudeStatus,
   Config,
   Dims,
@@ -218,7 +219,9 @@ export function createSessionManager(
    * H2/H3: a Claude Code hook reported activity for `sessionId`. Update the
    * status, append a timeline event (A4) when `eventClass` is supplied and the
    * timeline is enabled, and broadcast a `status` frame — carrying `gate` (B4)
-   * when the held approval is a 'tool'/'plan' gate. No-op for an unknown session.
+   * when the held approval is a 'tool'/'plan' gate, and `preview` (W1) when a
+   * bounded command/diff preview of the held tool was derived. No-op for an
+   * unknown session.
    */
   function handleHookEvent(
     sessionId: string,
@@ -228,6 +231,7 @@ export function createSessionManager(
     gate?: PermissionGate,
     eventClass?: string,
     toolName?: string,
+    preview?: ApprovalPreview,
   ): void {
     const session = sessions.get(sessionId);
     if (session === undefined) return;
@@ -246,6 +250,7 @@ export function createSessionManager(
     if (detail !== undefined) msg.detail = detail;
     if (pending) msg.pending = true;
     if (gate !== undefined) msg.gate = gate;
+    if (preview !== undefined) msg.preview = preview;
     broadcast(session, msg);
   }