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.

src/agents/pi-embedded-runner/google.ts

@@ -1,3 +1,5 @@
+import { EventEmitter } from "node:events";
+
 import type { AgentMessage, AgentTool } from "@mariozechner/pi-agent-core";
 import type { TSchema } from "@sinclair/typebox";
 import type { SessionManager } from "@mariozechner/pi-coding-agent";
@@ -184,10 +186,28 @@ export function logToolSchemasForGoogle(params: { tools: AgentTool[]; provider:
   }
 }
 
+// Event emitter for unhandled compaction failures that escape try-catch blocks.
+// Listeners can use this to trigger session recovery with retry.
+const compactionFailureEmitter = new EventEmitter();
+
+export type CompactionFailureListener = (reason: string) => void;
+
+/**
+ * Register a listener for unhandled compaction failures.
+ * Called when auto-compaction fails in a way that escapes the normal try-catch,
+ * e.g., when the summarization request itself exceeds the model's token limit.
+ * Returns an unsubscribe function.
+ */
+export function onUnhandledCompactionFailure(cb: CompactionFailureListener): () => void {
+  compactionFailureEmitter.on("failure", cb);
+  return () => compactionFailureEmitter.off("failure", cb);
+}
+
 registerUnhandledRejectionHandler((reason) => {
   const message = describeUnknownError(reason);
   if (!isCompactionFailureError(message)) return false;
   log.error(`Auto-compaction failed (unhandled): ${message}`);
+  compactionFailureEmitter.emit("failure", message);
   return true;
 });