perf: incremental transcript updates via WebSocket, eliminate polling

- Replace 1500ms polling with fs.watch recursive (reliable on Windows)
- Enrich WS broadcast with newContent delta + session metadata
- Client appends incrementally instead of 2 sequential HTTP requests
- Pre-initialize Tauri HTTP plugin at module load to avoid dynamic import overhead
- Per-file debounce timers (150ms) instead of single shared timer
- Size-based validation for safe incremental appends with HTTP fallback

frontend/src/lib/tauri.ts

@@ -17,6 +17,9 @@ export function isMobileTauri(): boolean {
 // Server URL storage (in-memory, loaded from Tauri store on init)
 let _serverUrl = ''
 
+// Cached Tauri HTTP fetch function (avoid dynamic import on every request)
+let _tauriFetch: typeof fetch | null = null
+
 export function getServerUrl(): string {
   return _serverUrl
 }
@@ -55,10 +58,13 @@ export async function apiFetch(input: string | URL | Request, init?: RequestInit
   // Resolve URL
   const url = typeof input === 'string' ? resolveUrl(input) : input
 
-  // Use Tauri HTTP plugin for cross-origin requests
+  // Use Tauri HTTP plugin for cross-origin requests (cached after first load)
   try {
-    const { fetch: tauriFetch } = await import('@tauri-apps/plugin-http')
-    return tauriFetch(url, init)
+    if (!_tauriFetch) {
+      const { fetch: tauriFetch } = await import('@tauri-apps/plugin-http')
+      _tauriFetch = tauriFetch
+    }
+    return _tauriFetch(url, init)
   } catch (e) {
     // Fallback to native fetch if plugin fails
     console.warn('[Tauri] HTTP plugin failed, falling back to fetch:', e)
@@ -66,6 +72,26 @@ export async function apiFetch(input: string | URL | Request, init?: RequestInit
   }
 }
 
+/**
+ * Eagerly initialize the Tauri HTTP plugin to avoid dynamic import overhead
+ * on the first apiFetch call. Call this once at app startup.
+ */
+export async function initTauriFetch() {
+  if (!isTauri || _tauriFetch) return
+  try {
+    const { fetch: tauriFetch } = await import('@tauri-apps/plugin-http')
+    _tauriFetch = tauriFetch
+    console.log('[Tauri] HTTP plugin pre-initialized')
+  } catch (e) {
+    console.warn('[Tauri] Failed to pre-init HTTP plugin:', e)
+  }
+}
+
+// Auto-init on module load in Tauri mode
+if (isTauri) {
+  initTauriFetch()
+}
+
 // Dynamic plugin imports (only used behind isTauri checks)
 export async function getTauriStore() {
   const { LazyStore } = await import('@tauri-apps/plugin-store')