Add auto-reconnect with exponential backoff

Sessions configured with auto-reconnect will automatically attempt to
re-establish the connection when unexpectedly disconnected. Uses
exponential backoff (1s, 2s, 4s, 8s... up to 30s) with a configurable
maximum number of attempts (default 5).

- New SessionReconnecting state with pulsing indicator in tab bar
- SSH and Telnet profiles gain autoReconnect and maxReconnectTries fields
- RAW protocol already had autoReconnect; now uses the shared mechanism
- CancelReconnect API to abort in-progress reconnection
- Frontend shows reconnect attempt notifications
- Profile editor exposes toggle and max attempts in Advanced/Reconnection

frontend/src/js/app.js

@@ -134,10 +134,21 @@ class App {
             this.terminals.handleData(payload.sessionID, payload.data);
         });
 
-        // Session state transitions (connecting → connected → disconnected)
+        // Session state transitions (connecting → connected → reconnecting → disconnected)
         window.runtime.EventsOn('session:state', (payload) => {
             this.tabs.updateSessionState(payload.sessionID, payload.state, payload.error);
             this.updateStatusBar();
+
+            // Show reconnecting notification
+            if (payload.state === 'reconnecting') {
+                const attempt = payload.attempt || 1;
+                const maxRetries = payload.maxRetries || 5;
+                const delaySec = payload.delaySec || 1;
+                this.notifications.info(
+                    `Reconnecting (attempt ${attempt}/${maxRetries}) in ${delaySec}s…`
+                );
+            }
+
             // Once connected, ensure the backend PTY has the correct dimensions.
             // The initial fit happens before the SSH handshake completes, so the
             // onResize callback fires when the backend isn't ready. By the time