fix: improve terminal robustness with exponential backoff and better keepalive

- Backend: Add 10s connection timeout, improve SSH keepalive (15s × 8 retries)
- Backend: Add detailed error logging for debugging
- Frontend: Implement exponential backoff (1s → 30s max) instead of fixed 2s
- Frontend: Reduce heartbeat to 12s (within 15s SSH keepalive window)
- Frontend: Reset reconnect counter on successful connection
- Frontend: Fix race condition in auth recovery after manual close

dashboard/frontend/src/routes/Terminal.svelte

@@ -194,6 +194,8 @@
       let closedManually = false;
       let reconnecting = false;
       let authRecoveryInFlight = false;
+      let reconnectAttempts = 0;
+      const MAX_RECONNECT_DELAY = 30000;
 
       function clearHeartbeat() {
         if (heartbeat) {
@@ -229,17 +231,19 @@
         clearHeartbeat();
         if (closedManually || reconnectTimer) return;
         reconnecting = true;
+        reconnectAttempts++;
+        const delay = Math.min(1000 * Math.pow(2, reconnectAttempts - 1), MAX_RECONNECT_DELAY);
         updateTab(tabId, {
           connected: false,
-          error: `${reason} — reconnecting...`,
+          error: `${reason} — reconnecting in ${Math.round(delay / 1000)}s (attempt ${reconnectAttempts})...`,
           statusBanner: tab.persistent ? "Reconnecting to persistent session..." : "",
         });
-        term.write(`\r\n\x1b[90m[${reason} — reconnecting...]\x1b[0m`);
+        term.write(`\r\n\x1b[90m[${reason} — reconnecting in ${Math.round(delay / 1000)}s...]\x1b[0m`);
         reconnectTimer = setTimeout(() => {
           reconnectTimer = null;
           if (!tabData.has(tabId) || closedManually) return;
           connect(true);
-        }, 2000);
+        }, delay);
       }
 
       function sendSize(ws) {
@@ -267,6 +271,7 @@
         ws.binaryType = "arraybuffer";
 
         ws.onopen = () => {
+          reconnectAttempts = 0;
           updateTab(tabId, {
             connected: true,
             error: "",
@@ -276,7 +281,7 @@
           clearHeartbeat();
           heartbeat = setInterval(() => {
             if (ws.readyState === 1) ws.send("__ping__");
-          }, 25000);
+          }, 12000);
           const d = tabData.get(tabId);
           if (d) {
             d.ws = ws;
@@ -311,7 +316,7 @@
               authRecoveryInFlight = true;
               const refreshed = await tryRefreshSession();
               authRecoveryInFlight = false;
-              if (refreshed) {
+              if (refreshed && !closedManually) {
                 reconnecting = true;
                 updateTab(tabId, {
                   connected: false,