Add keyboard shortcuts system with reusable hint components

- Implement centralized keyboard registry with 20+ shortcuts
- Add instance navigation (Cmd+[1-9], Cmd+[/])
- Add session navigation (Cmd+Shift+[1-9], Cmd+Shift+[/])
- Add agent/model cycling (Tab, Cmd+Shift+M)
- Add input shortcuts (Cmd+P focus, Cmd+K clear, ↑↓ history)
- Add command palette (Cmd+Shift+P) with 8 MVP commands
- Implement message history per folder in IndexedDB (max 100)
- Create reusable Kbd and HintRow components
- Replace all keyboard hint rendering with consistent components
- Use text-based shortcuts (Cmd+Shift+M) for clarity

src/stores/command-palette.ts

@@ -0,0 +1,17 @@
+import { createSignal } from "solid-js"
+
+const [isOpen, setIsOpen] = createSignal(false)
+
+export function showCommandPalette() {
+  setIsOpen(true)
+}
+
+export function hideCommandPalette() {
+  setIsOpen(false)
+}
+
+export function toggleCommandPalette() {
+  setIsOpen(!isOpen())
+}
+
+export { isOpen }