fix: queue simultaneous approval requests per session (fixes #527)

Changes _pending from a single overwriting dict value to a list,
so parallel tool calls each get their own approval slot.

api/routes.py:
- Wraps submit_pending() to append to a list and assign a stable
  approval_id (uuid4) to each entry.
- _handle_approval_pending() returns the first queued entry plus
  pending_count so the UI can show '1 of N'.
- _handle_approval_respond() pops by approval_id (falls back to
  oldest entry for backward-compat with old clients).
- Backward-compat: legacy single-dict values in _pending are
  handled without crashing.

static/messages.js:
- respondApproval() sends approval_id in the POST body.
- showApprovalCard() accepts pendingCount, shows '1 of N pending'
  counter when multiple approvals are queued.
- _approvalCurrentId tracks the approval_id of the displayed card.
- Poll loop passes pending_count to showApprovalCard.

static/index.html:
- Adds approvalCounter element for the '1 of N' display.

tests/test_approval_queue.py:
- 14 tests: static-analysis checks (Python + JS + HTML),
  functional tests that inject two simultaneous approvals and
  verify both are surfaced and independently resolvable.

static/index.html

@@ -225,6 +225,7 @@
         </div>
         <div class="approval-desc" id="approvalDesc"></div>
         <div class="approval-cmd" id="approvalCmd"></div>
+        <div class="approval-counter" id="approvalCounter" style="display:none;font-size:0.75em;opacity:0.6;margin-top:4px;"></div>
         <div class="approval-btns">
           <button class="approval-btn once" id="approvalBtnOnce" onclick="respondApproval('once')" title="Allow this one command (Enter)" data-i18n-title="approval_btn_once_title">
             <span class="approval-btn-icon"><svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2.5" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><polyline points="20 6 9 17 4 12"/></svg></span>