perf: Phase 1 critical performance fixes + production macOS build

Performance:
- Move JSON parsing to background isolate via Isolate.run() (eliminates 2-4s UI freeze)
- Cache filteredEntries with key-based invalidation (eliminates O(n) recomputation)
- Debounce search queries at 300ms (prevents cascade rebuilds on keystroke)
- Flatten timeline from 2-level turn×response Column to single virtualized ListView.builder
- Add RepaintBoundary per timeline item (isolates repaints during scroll)
- Use context.select for granular rebuilds instead of top-level context.watch
- Lazy ExpandableCard: child not built until first expand (replaces AnimatedCrossFade)
- Use IndexedStack in AppShell (preserves screen state across tab switches)

Fixes:
- Collect parse errors instead of silently swallowing them
- Show parse error count in timeline filter bar
- Fix overflow in tokens screen pie chart legend

Build:
- Configure Developer ID signing with hardened runtime for production distribution
- Enable secure timestamps for notarization
- Update app name to Claude Session Viewer
- Signed, notarized, stapled DMG distribution

lib/widgets/navigation/app_shell.dart

@@ -39,24 +39,18 @@ class _AppShellState extends State<AppShell> {
           hasSession: hasSession,
         ),
         Expanded(
-          child: _buildScreen(),
+          child: IndexedStack(
+            index: _screen.index,
+            children: [
+              HomeScreen(onSessionLoaded: _onSessionLoaded),
+              const TimelineScreen(),
+              const AgentsScreen(),
+              const ToolbeltScreen(),
+              const TokensScreen(),
+            ],
+          ),
         ),
       ],
     );
   }
-
-  Widget _buildScreen() {
-    switch (_screen) {
-      case SidebarScreen.home:
-        return HomeScreen(onSessionLoaded: _onSessionLoaded);
-      case SidebarScreen.timeline:
-        return const TimelineScreen();
-      case SidebarScreen.agents:
-        return const AgentsScreen();
-      case SidebarScreen.toolbelt:
-        return const ToolbeltScreen();
-      case SidebarScreen.tokens:
-        return const TokensScreen();
-    }
-  }
 }