Fix layout, terminal sizing, notifications, and sidebar styling

Layout:
- Add panel-container and session-panel CSS so workspace content
  doesn't overlap sidebar
- Session panels use position: absolute to fill workspace area
- Workspace is now flex column for proper empty-state/panel stacking

Terminal:
- xterm.js overrides now target .session-panel in addition to
  .terminal-container so terminal fills available space
- Added debounced window resize handler for terminal re-fit

Notifications:
- Rewrote Notifications class to emit .notification-* classes matching
  the existing polished CSS (was emitting .toast-* with no CSS)
- Icon is now a small coloured circle badge instead of oversized SVG

Sidebar:
- Fixed class mismatches: sidebar-dot → protocol-indicator,
  sidebar-item-sub → sidebar-item-host, sidebar-star → sidebar-item-star
- protocolColour() now returns proto-ssh etc. matching CSS selectors

Other:
- Replaced prompt() in tab rename with custom _inputPrompt modal

frontend/src/js/app.js

@@ -295,8 +295,9 @@ class App {
             searchInput.addEventListener('input', (e) => this.sidebar.search(e.target.value));
         }
 
-        // Window resize → re-fit active terminal
-        window.addEventListener('resize', () => this.terminals.fitActive());
+        // Window resize → re-fit active terminal (debounced)
+        const { debounce } = window.NexusTerminal;
+        window.addEventListener('resize', debounce(() => this.terminals.fitActive(), 100));
     }
 
     // ── Connection lifecycle ─────────────────────────────────────────