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/tabs.js

@@ -446,10 +446,12 @@ class TabManager {
     _promptRename(sessionId) {
         const session = this.app.state.getSession(sessionId);
         const current = session?.tabName || '';
-        const name = prompt('Tab name:', current);
-        if (name !== null && name.trim()) {
-            this.renameTab(sessionId, name.trim());
-        }
+        // Use the app's custom prompt modal (window.prompt is unavailable in WKWebView)
+        this.app._inputPrompt('Rename Tab', 'Tab name', current).then((name) => {
+            if (name !== null && name.trim()) {
+                this.renameTab(sessionId, name.trim());
+            }
+        });
     }
 
     /**