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/sidebar.js

@@ -151,7 +151,7 @@ class Sidebar {
         }
 
         // Protocol colour dot
-        const dot = createElement('span', `sidebar-dot ${protocolColour(profile.protocol)}`);
+        const dot = createElement('span', `protocol-indicator ${protocolColour(profile.protocol)}`);
         item.appendChild(dot);
 
         // Text content
@@ -164,7 +164,7 @@ class Sidebar {
         const subtitle = profile.host
             ? `${profile.host}:${profile.port}`
             : profile.protocol.toUpperCase();
-        const subEl = createElement('span', 'sidebar-item-sub', {
+        const subEl = createElement('span', 'sidebar-item-host', {
             textContent: truncate(subtitle, 30),
         });
         textWrapper.appendChild(subEl);
@@ -172,7 +172,7 @@ class Sidebar {
 
         // Favourite star
         if (profile.favourite) {
-            const star = createElement('span', 'sidebar-star', { textContent: '★' });
+            const star = createElement('span', 'sidebar-item-star is-favourite', { textContent: '★' });
             item.appendChild(star);
         }