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/lib/keyboard-utils.ts

@@ -0,0 +1,30 @@
+import type { KeyboardShortcut } from "./keyboard-registry"
+
+export const isMac = () => navigator.platform.toLowerCase().includes("mac")
+
+export const modKey = (event?: KeyboardEvent) => {
+  if (!event) return isMac() ? "metaKey" : "ctrlKey"
+  return isMac() ? event.metaKey : event.ctrlKey
+}
+
+export const modKeyPressed = (event: KeyboardEvent) => {
+  return isMac() ? event.metaKey : event.ctrlKey
+}
+
+export const formatShortcut = (shortcut: KeyboardShortcut): string => {
+  const parts: string[] = []
+
+  if (shortcut.modifiers.ctrl || shortcut.modifiers.meta) {
+    parts.push(isMac() ? "Cmd" : "Ctrl")
+  }
+  if (shortcut.modifiers.shift) {
+    parts.push("Shift")
+  }
+  if (shortcut.modifiers.alt) {
+    parts.push(isMac() ? "Option" : "Alt")
+  }
+
+  parts.push(shortcut.key.toUpperCase())
+
+  return parts.join("+")
+}