feat(ui): add Doctor page (4th destination) — full health snapshot

Composes the platform's health picture in one view:
- Summary strip: 4 stat tiles for modules / approvals / audit
  chain / services with semantic accent colour.
- Event-log panel: WORM-1 badge + chain status, points to the
  first tampered event id when integrity fails.
- Modules & approvals panel: counts plus an "attention" pill
  when approvals are pending.
- Services panel: declared services from operator config or a
  helpful empty-state pointing at ~/.fai/config.yaml.

Backed by the new HubAdmin RPCs (VerifyEventChain, ListServices)
plus the existing ListCapabilities and ListApprovals. One
HubService.doctor() call fans out to all four in parallel.

Doctor is the new first destination in the sidebar — that's the
page an operator wants to land on after starting the app.

Bumps fai_studio 0.3.0 -> 0.4.0.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/main.dart

@@ -10,6 +10,7 @@ import 'package:flutter/material.dart';
 import 'data/hub.dart';
 import 'pages/approvals.dart';
 import 'pages/audit.dart';
+import 'pages/doctor.dart';
 import 'pages/modules.dart';
 import 'theme/theme.dart';
 import 'theme/tokens.dart';
@@ -48,6 +49,12 @@ class _StudioShellState extends State<StudioShell> {
   Timer? _healthPoll;
 
   static const _pages = <_NavPage>[
+    _NavPage(
+      label: 'Doctor',
+      icon: Icons.health_and_safety_outlined,
+      selectedIcon: Icons.health_and_safety,
+      page: DoctorPage(),
+    ),
     _NavPage(
       label: 'Modules',
       icon: Icons.extension_outlined,