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/common/expandable_card.dart

@@ -26,11 +26,13 @@ class _ExpandableCardState extends State<ExpandableCard>
   late bool _expanded;
   late AnimationController _controller;
   late Animation<double> _rotation;
+  bool _hasBeenExpanded = false; // Track if ever expanded to lazy-build
 
   @override
   void initState() {
     super.initState();
     _expanded = widget.initiallyExpanded;
+    _hasBeenExpanded = _expanded;
     _controller = AnimationController(
       duration: const Duration(milliseconds: 200),
       vsync: this,
@@ -51,6 +53,7 @@ class _ExpandableCardState extends State<ExpandableCard>
     setState(() {
       _expanded = !_expanded;
       if (_expanded) {
+        _hasBeenExpanded = true;
         _controller.forward();
       } else {
         _controller.reverse();
@@ -76,7 +79,8 @@ class _ExpandableCardState extends State<ExpandableCard>
             onTap: _toggle,
             borderRadius: BorderRadius.circular(8),
             child: Padding(
-              padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 10),
+              padding:
+                  const EdgeInsets.symmetric(horizontal: 12, vertical: 10),
               child: Row(
                 children: [
                   RotationTransition(
@@ -93,19 +97,18 @@ class _ExpandableCardState extends State<ExpandableCard>
               ),
             ),
           ),
-          ClipRect(
-            child: AnimatedCrossFade(
-              firstChild: const SizedBox.shrink(),
-              secondChild: Padding(
-                padding: const EdgeInsets.fromLTRB(12, 0, 12, 12),
-                child: widget.child,
+          // Lazy animated expand: only build child once ever expanded
+          if (_hasBeenExpanded)
+            ClipRect(
+              child: SizeTransition(
+                sizeFactor: _controller,
+                axisAlignment: -1.0,
+                child: Padding(
+                  padding: const EdgeInsets.fromLTRB(12, 0, 12, 12),
+                  child: widget.child,
+                ),
               ),
-              crossFadeState: _expanded
-                  ? CrossFadeState.showSecond
-                  : CrossFadeState.showFirst,
-              duration: const Duration(milliseconds: 200),
             ),
-          ),
         ],
       ),
     );