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/css/tabs.css

@@ -125,9 +125,17 @@
 
 .tab-status.is-connected    { background: var(--success); }
 .tab-status.is-connecting   { background: var(--warning); animation: pulse-dot 1.2s ease infinite; }
+.tab-status.is-reconnecting { background: var(--warning); animation: pulse-dot 0.8s ease infinite; }
 .tab-status.is-error        { background: var(--error); }
 .tab-status.is-disconnected { background: var(--text-tertiary); }
 
+/* Also match the status-* pattern used by updateSessionState */
+.tab-status.status-connected    { background: var(--success); }
+.tab-status.status-connecting   { background: var(--warning); animation: pulse-dot 1.2s ease infinite; }
+.tab-status.status-reconnecting { background: var(--warning); animation: pulse-dot 0.8s ease infinite; }
+.tab-status.status-error        { background: var(--error); }
+.tab-status.status-disconnected { background: var(--text-tertiary); }
+
 /* -----------------------------------------------------------------------
    SFTP toggle button — revealed on hover (SSH tabs only)
    ----------------------------------------------------------------------- */