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/blocks/extract-blocks.ts

@@ -5,6 +5,23 @@ export interface ExtractedBlock {
   content: string
 }
 
+function escapeRegExp(value: string): string {
+  return value.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')
+}
+
+/** Contenu texte d'un bloc identifié par data-id (sans filtre longueur min). */
+export function extractBlockContentById(html: string, blockId: string): string | null {
+  if (!html || !blockId) return null
+  const regex = new RegExp(
+    `<(?:p|h[1-6]|blockquote|li)[^>]*data-id="${escapeRegExp(blockId)}"[^>]*>([\\s\\S]*?)<\\/(?:p|h[1-6]|blockquote|li)>`,
+    'i',
+  )
+  const match = regex.exec(html)
+  if (!match) return null
+  const content = stripHtmlToPlainText(match[1])
+  return content.length > 0 ? content : null
+}
+
 export function extractBlocksFromHtml(html: string): ExtractedBlock[] {
   const blocks: ExtractedBlock[] = []
   const regex = /<(?:p|h[1-6]|blockquote|li)[^>]*data-id="([^"]+)"[^>]*>([\s\S]*?)<\/(?:p|h[1-6]|blockquote|li)>/gi