fix: terminal resize on tab switch + flickering from activity marking

Resize fix:
- ResizeObserver now checks contentRect dimensions before fitting
- Ignores resize events when container width/height < 50px (hidden tab)
- Prevents xterm.js from calculating 8-char columns on zero-width containers

Flickering fix:
- markActivity throttled to once per second per session
- Was firing on every single data event (hundreds per second during
  active output), triggering Vue reactivity updates on the sessions
  array, causing tab bar and session container re-renders

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/composables/useTerminal.ts

@@ -163,9 +163,16 @@ export function useTerminal(sessionId: string, backend: 'ssh' | 'pty' = 'ssh'):
     // Subscribe to SSH output events for this session.
     // Tauri v2 listen() callback receives { payload: T } — the base64 string
     // is in event.payload (not event.data as in Wails).
+    // Throttle activity marking to avoid Vue reactivity storms
+    let lastActivityMark = 0;
+
     unlistenPromise = listen<string>(dataEvent, (event) => {
-      // Mark tab activity for background sessions
-      try { useSessionStore().markActivity(sessionId); } catch {}
+      // Mark tab activity at most once per second
+      const now = Date.now();
+      if (now - lastActivityMark > 1000) {
+        lastActivityMark = now;
+        try { useSessionStore().markActivity(sessionId); } catch {}
+      }
 
       const b64data = event.payload;
 
@@ -193,9 +200,12 @@ export function useTerminal(sessionId: string, backend: 'ssh' | 'pty' = 'ssh'):
       unlistenFn = fn;
     });
 
-    // Auto-fit when the container resizes
-    resizeObserver = new ResizeObserver(() => {
-      fitAddon.fit();
+    // Auto-fit when the container resizes — but only if visible
+    resizeObserver = new ResizeObserver((entries) => {
+      const entry = entries[0];
+      if (entry && entry.contentRect.width > 50 && entry.contentRect.height > 50) {
+        fitAddon.fit();
+      }
     });
     resizeObserver.observe(container);
   }