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>

docs/sprint-status.yaml

@@ -58,3 +58,7 @@ development_status:
   4-5-eu-data-residency: backlog
   4-6-sso-saml-audit-logging: backlog
   epic-4-retrospective: optional
+  epic-5: in-progress
+  5-1-nextgen-editor: ready-for-dev
+
+