feat: add auto-memory panel, /memory command, and unified toast system

- Add /memory CLI built-in slash command
- Refactor MemoryBrowserPanel to accept isOpen/onClose props
- Add Send /memory and Refresh buttons to MemoryBrowserPanel header
- Add Memory Manager entry to NavMenu
- Create unified ToastContainer replacing AchievementNotification and
  UpdateNotification with a single stacked toast system
- Add toasts store with info (4s), achievement (5s), and persistent
  update toast types
- Move getAchievementRarity and getRarityColour helpers to toasts store
- Detect auto-memory writes in tauri.ts output listener and fire toast
- Remove action buttons from update toast; version is now a direct link

Closes #212

src/lib/commands/slashCommands.test.ts

@@ -92,10 +92,11 @@ describe("slashCommands", () => {
       expect(commandNames).toContain("simplify");
       expect(commandNames).toContain("loop");
       expect(commandNames).toContain("batch");
+      expect(commandNames).toContain("memory");
     });
 
-    it("has 10 commands total", () => {
-      expect(slashCommands.length).toBe(10);
+    it("has 11 commands total", () => {
+      expect(slashCommands.length).toBe(11);
     });
 
     it("each command has required properties", () => {