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/layout.css

@@ -70,6 +70,24 @@
     position: relative;
     overflow: hidden;
     background: var(--bg-base);
+    display: flex;
+    flex-direction: column;
+}
+
+/* Panel container — fills workspace, holds session panels */
+.panel-container {
+    flex: 1;
+    position: relative;
+    overflow: hidden;
+}
+
+/* Individual session panel — one per tab, fills the panel container */
+.session-panel {
+    position: absolute;
+    inset: 0;
+    display: flex;
+    flex-direction: column;
+    overflow: hidden;
 }
 
 /* Terminal container — fills workspace */