feat: editor improvements and architectural grid prototype

Multiple feature additions and improvements across the application:

- NextGen Editor: drag handles, smart paste, block actions
- Structured views: Kanban and table layouts for notes
- Architectural Grid: new brainstorming/agent interface prototype
- Flashcards: SM-2 revision algorithm with AI generation
- MCP server: robustness improvements
- Graph/PDF chat: fix click propagation and copy behavior
- Various UI/UX enhancements and bug fixes

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

memento-note/lib/use-hydrated.ts

@@ -0,0 +1,10 @@
+import { useSyncExternalStore } from 'react'
+
+/** True only after the client has hydrated (false on server + first client paint). */
+export function useHydrated(): boolean {
+  return useSyncExternalStore(
+    () => () => {},
+    () => true,
+    () => false,
+  )
+}