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/css/components.css

@@ -623,11 +623,24 @@
 
 .notification-icon {
     flex-shrink: 0;
-    font-size: var(--text-lg);
+    width: 20px;
+    height: 20px;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    border-radius: 50%;
+    font-size: 11px;
+    font-weight: var(--weight-bold);
     line-height: 1;
-    margin-top: 1px;
+    color: var(--text-inverse);
+    background: var(--text-tertiary);
 }
 
+.notification-success .notification-icon { background: var(--success); }
+.notification-error .notification-icon   { background: var(--error); }
+.notification-warning .notification-icon { background: var(--warning); }
+.notification-info .notification-icon    { background: var(--info); }
+
 .notification-body {
     flex: 1;
     min-width: 0;
@@ -665,16 +678,9 @@
 
 /* Colour variants (left border accent) */
 .notification-success { border-left: 3px solid var(--success); }
-.notification-success .notification-icon { color: var(--success); }
-
-.notification-error { border-left: 3px solid var(--error); }
-.notification-error .notification-icon { color: var(--error); }
-
+.notification-error   { border-left: 3px solid var(--error); }
 .notification-warning { border-left: 3px solid var(--warning); }
-.notification-warning .notification-icon { color: var(--warning); }
-
-.notification-info { border-left: 3px solid var(--info); }
-.notification-info .notification-icon { color: var(--info); }
+.notification-info    { border-left: 3px solid var(--info); }
 
 
 /* =======================================================================
@@ -790,7 +796,7 @@
     flex-direction: column;
     align-items: center;
     justify-content: center;
-    height: 100%;
+    flex: 1;
     padding: var(--space-8);
     text-align: center;
 }