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/backend/routers/terminal.py

@@ -125,13 +125,22 @@ async def ws_endpoint(websocket: WebSocket, host: str = Query("nas"), token: str
 
     h = HOSTS[host]
     try:
-        conn = await asyncssh.connect(
-            h["host"], username=h["user"],
-            client_keys=[h["key"]], known_hosts=_known_hosts,
-            keepalive_interval=30,
-            keepalive_count_max=3,
+        conn = await asyncio.wait_for(
+            asyncssh.connect(
+                h["host"], username=h["user"],
+                client_keys=[h["key"]], known_hosts=_known_hosts,
+                keepalive_interval=15,
+                keepalive_count_max=8,
+            ),
+            timeout=10.0
         )
-    except Exception:
+    except asyncio.TimeoutError:
+        logger.error("SSH connection to %s timed out after 10s", host)
+        await _release_session(session_id)
+        await websocket.close(code=1011, reason="SSH connection timeout")
+        return
+    except Exception as e:
+        logger.error("SSH connection to %s failed: %s", host, e)
         await _release_session(session_id)
         await websocket.close(code=1011, reason="SSH connection failed")
         return
@@ -149,8 +158,12 @@ async def ws_endpoint(websocket: WebSocket, host: str = Query("nas"), token: str
                     if not data:
                         break
                     await websocket.send_bytes(data)
-            except (asyncssh.Error, asyncio.CancelledError):
+            except asyncssh.Error as e:
+                logger.warning("SSH read error for %s: %s", host, e)
+            except asyncio.CancelledError:
                 pass
+            except Exception as e:
+                logger.error("Unexpected error reading SSH for %s: %s", host, e)
 
         read_task = asyncio.create_task(read_ssh())
 
@@ -169,8 +182,8 @@ async def ws_endpoint(websocket: WebSocket, host: str = Query("nas"), token: str
                     if msg["text"] == "__ping__":
                         continue
                     process.stdin.write(msg["text"].encode())
-        except Exception:
-            pass
+        except Exception as e:
+            logger.info("WebSocket receive loop ended for %s: %s", host, e)
         finally:
             read_task.cancel()
             process.close()