feat: redesign activity panel UX with grouped timeline and verb-based labels

Replace the flat, repetitive 'Agent used X' activity list with a
structured narrative timeline:

- Group consecutive same-tool calls into collapsible rows with context
  (e.g. 'Viewed 4 files' with file names listed below)
- Use verb-based labels with arguments: 'Viewed ChatPane.tsx:148-250',
  'Searched for tool-call', 'Edited runTimeline.ts'
- Promote report_intent events to phase dividers that segment the
  timeline into labeled stages of work
- Show latest intent text in the collapsed header summary
- Use category-specific icons (Eye, Search, Pencil, Terminal, etc.)
  instead of the universal wrench
- Render thinking steps as quoted blocks with 'N more' toggle for
  consecutive groups

All data was already available in RunTimelineEventRecord.toolArguments;
this change surfaces it prominently instead of hiding it behind
expandable detail panels.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/styles.css

@@ -663,6 +663,17 @@ body {
   animation: turn-activity-row-in 0.15s cubic-bezier(0.16, 1, 0.3, 1) both;
 }
 
+@keyframes intent-divider-in {
+  from {
+    opacity: 0;
+    transform: scaleX(0.7);
+  }
+}
+
+.turn-activity-row[role="separator"] {
+  animation: intent-divider-in 0.2s cubic-bezier(0.16, 1, 0.3, 1) both;
+}
+
 /* ── Respect reduced motion ──────────────────────────────────── */
 
 @media (prefers-reduced-motion: reduce) {
@@ -677,7 +688,8 @@ body {
   .banner-slide-enter,
   .update-banner-enter,
   .turn-activity-enter,
-  .turn-activity-row {
+  .turn-activity-row,
+  .turn-activity-row[role="separator"] {
     animation: none;
   }
 }