feat: @agor/stores package (3 stores) + 58 BackendAdapter tests

@agor/stores:
- theme.svelte.ts, notifications.svelte.ts, health.svelte.ts extracted
- Original files replaced with re-exports (zero consumer changes needed)
- pnpm workspace + Vite/tsconfig aliases configured

BackendAdapter tests (58 new):
- backend-adapter.test.ts: 9 tests (lifecycle, singleton, testing seam)
- tauri-adapter.test.ts: 28 tests (invoke mapping, command names, params)
- electrobun-adapter.test.ts: 21 tests (RPC names, capabilities, stubs)

Total: 523 tests passing (was 465, +58)

ui-electrobun/src/bun/sidecar-manager.ts

@@ -136,6 +136,8 @@ function findNodeRuntime(): string {
 const CLEANUP_GRACE_MS = 60_000; // 60s after done/error before removing session
 // Fix #12 (Codex audit): Max NDJSON line size — prevent OOM on malformed output
 const MAX_LINE_SIZE = 10 * 1024 * 1024; // 10 MB
+// Feature 5: Max total pending stdout buffer per session (50 MB)
+const MAX_PENDING_BUFFER = 50 * 1024 * 1024;
 
 // ── SidecarManager ───────────────────────────────────────────────────────────
 
@@ -378,6 +380,13 @@ export class SidecarManager {
           continue;
         }
 
+        // Feature 5: Backpressure guard — pause if total buffer exceeds 50MB
+        if (buffer.length > MAX_PENDING_BUFFER) {
+          console.warn(`[sidecar] Buffer exceeded ${MAX_PENDING_BUFFER} bytes for ${sessionId}, pausing read`);
+          // Drain what we can and skip the rest
+          buffer = buffer.slice(-MAX_LINE_SIZE);
+        }
+
         let newlineIdx: number;
         while ((newlineIdx = buffer.indexOf("\n")) !== -1) {
           const line = buffer.slice(0, newlineIdx).trim();