feat(v2): add tiling layout, sidebar controls, and keyboard shortcuts

- TilingGrid: dynamic CSS Grid with auto-preset based on pane count
- Layout presets: 1-col, 2-col, 3-col, 2x2, master-stack
- PaneContainer: close button, status indicator, focus highlight
- SessionList: new terminal button, layout preset selector, pane list
- Layout store: pane CRUD, focus management, grid template generation
- Keyboard: Ctrl+N new terminal, Ctrl+1-4 focus pane by index

v2/src/App.svelte

@@ -1,6 +1,39 @@
 <script lang="ts">
+  import { onMount } from 'svelte';
   import SessionList from './lib/components/Sidebar/SessionList.svelte';
   import TilingGrid from './lib/components/Layout/TilingGrid.svelte';
+  import { addPane, focusPaneByIndex, getPanes } from './lib/stores/layout';
+
+  function newTerminal() {
+    const id = crypto.randomUUID();
+    const num = getPanes().length + 1;
+    addPane({
+      id,
+      type: 'terminal',
+      title: `Terminal ${num}`,
+    });
+  }
+
+  onMount(() => {
+    function handleKeydown(e: KeyboardEvent) {
+      // Ctrl+N — new terminal
+      if (e.ctrlKey && !e.shiftKey && e.key === 'n') {
+        e.preventDefault();
+        newTerminal();
+        return;
+      }
+
+      // Ctrl+1-4 — focus pane by index
+      if (e.ctrlKey && !e.shiftKey && e.key >= '1' && e.key <= '4') {
+        e.preventDefault();
+        focusPaneByIndex(parseInt(e.key) - 1);
+        return;
+      }
+    }
+
+    window.addEventListener('keydown', handleKeydown);
+    return () => window.removeEventListener('keydown', handleKeydown);
+  });
 </script>
 
 <aside class="sidebar">