feat(compaction): add adaptive chunk sizing, progressive fallback, and UI indicator (#1466)

* fix(ui): allow relative URLs in avatar validation

The isAvatarUrl check only accepted http://, https://, or data: URLs,
but the /avatar/{agentId} endpoint returns relative paths like /avatar/main.
This caused local file avatars to display as text instead of images.

Fixes avatar display for locally configured avatar files.

* fix(gateway): resolve local avatars to URL in HTML injection and RPC

The frontend fix alone wasn't enough because:
1. serveIndexHtml() was injecting the raw avatar filename into HTML
2. agent.identity.get RPC was returning raw filename, overwriting the
   HTML-injected value

Now both paths resolve local file avatars (*.png, *.jpg, etc.) to the
/avatar/{agentId} endpoint URL.

* feat(compaction): add adaptive chunk sizing and progressive fallback

- Add computeAdaptiveChunkRatio() to reduce chunk size for large messages
- Add isOversizedForSummary() to detect messages too large to summarize
- Add summarizeWithFallback() with progressive fallback:
  - Tries full summarization first
  - Falls back to partial summarization excluding oversized messages
  - Notes oversized messages in the summary output
- Add SAFETY_MARGIN (1.2x) buffer for token estimation inaccuracy
- Reduce MIN_CHUNK_RATIO to 0.15 for very large messages

This prevents compaction failures when conversations contain
unusually large tool outputs or responses that exceed the
summarization model's context window.

* feat(ui): add compaction indicator and improve event error handling

Compaction indicator:
- Add CompactionStatus type and handleCompactionEvent() in app-tool-stream.ts
- Show '🧹 Compacting context...' toast while active (with pulse animation)
- Show '🧹 Context compacted' briefly after completion
- Auto-clear toast after 5 seconds
- Add CSS styles for .callout.info, .callout.success, .compaction-indicator

Error handling improvements:
- Wrap onEvent callback in try/catch in gateway.ts to prevent errors
  from breaking the WebSocket message handler
- Wrap handleGatewayEvent in try/catch with console.error logging
  to isolate errors and make them visible in devtools

These changes address UI freezes during heavy agent activity by:
1. Showing users when compaction is happening
2. Preventing uncaught errors from silently breaking the event loop

* fix(control-ui): add agentId to DEFAULT_ASSISTANT_IDENTITY

TypeScript inferred the union type without agentId when falling back to
DEFAULT_ASSISTANT_IDENTITY, causing build errors at control-ui.ts:222-223.

ui/src/styles/components.css

@@ -415,6 +415,51 @@
   color: var(--danger);
 }
 
+.callout.info {
+  border-color: rgba(92, 156, 255, 0.4);
+  color: var(--accent);
+}
+
+.callout.success {
+  border-color: rgba(92, 255, 128, 0.4);
+  color: var(--positive, #5cff80);
+}
+
+.compaction-indicator {
+  font-size: 13px;
+  padding: 8px 12px;
+  margin-bottom: 8px;
+  animation: compaction-fade-in 0.2s ease-out;
+}
+
+.compaction-indicator--active {
+  animation: compaction-pulse 1.5s ease-in-out infinite;
+}
+
+.compaction-indicator--complete {
+  animation: compaction-fade-in 0.2s ease-out;
+}
+
+@keyframes compaction-fade-in {
+  from {
+    opacity: 0;
+    transform: translateY(-4px);
+  }
+  to {
+    opacity: 1;
+    transform: translateY(0);
+  }
+}
+
+@keyframes compaction-pulse {
+  0%, 100% {
+    opacity: 0.7;
+  }
+  50% {
+    opacity: 1;
+  }
+}
+
 .code-block {
   font-family: var(--mono);
   font-size: 12px;