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/editor/copy-text-to-clipboard.ts

@@ -0,0 +1,30 @@
+/** Copie texte — writeText async + repli synchrone execCommand. */
+export async function copyTextToClipboard(text: string): Promise<boolean> {
+  if (typeof window === 'undefined') return false
+
+  if (navigator.clipboard?.writeText) {
+    try {
+      await navigator.clipboard.writeText(text)
+      return true
+    } catch {
+      // repli ci-dessous
+    }
+  }
+
+  try {
+    const ta = document.createElement('textarea')
+    ta.value = text
+    ta.setAttribute('readonly', '')
+    ta.style.position = 'fixed'
+    ta.style.left = '-9999px'
+    ta.style.top = '0'
+    document.body.appendChild(ta)
+    ta.focus()
+    ta.select()
+    const ok = document.execCommand('copy')
+    document.body.removeChild(ta)
+    return ok
+  } catch {
+    return false
+  }
+}