fix(electrobun): 7 bug fixes + 3 partial features completed

Bugs fixed:
- Agent retry: clear dead session on failed start (was non-recoverable)
- seqId persist: save seq_id column in agent_messages, migrate existing DBs
- Window frame save: wire resize event listener to saveWindowFrame()
- Diagnostics counters: real RPC call counter via withRpcCounting() proxy

Partial features completed:
- Search auto-seed: rebuildIndex() on startup + indexMessage() on save
- Notifications: removed 3 hardcoded demo entries, start empty
- Agent cost streaming: emitCostIfChanged() on every message batch

New: src/bun/rpc-stats.ts (RPC call + dropped event counters)

ui-electrobun/src/bun/rpc-stats.ts

@@ -0,0 +1,23 @@
+/**
+ * Simple RPC call counter for diagnostics.
+ * Incremented on each RPC request, read by diagnostics.stats handler.
+ */
+
+let _rpcCallCount = 0;
+let _droppedEvents = 0;
+
+export function incrementRpcCallCount(): void {
+  _rpcCallCount++;
+}
+
+export function incrementDroppedEvents(): void {
+  _droppedEvents++;
+}
+
+export function getRpcCallCount(): number {
+  return _rpcCallCount;
+}
+
+export function getDroppedEvents(): number {
+  return _droppedEvents;
+}