chore: add plan files for Phase 2 enhanced features

plans/phase-2-enhanced-features/CONTEXT.md

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+# Feature Context: Phase 2 — Enhanced Features
+
+## Current State
+MVP is complete and merged to master. All build/test/lint passes. 151 files, 115 tests.
+Starting Phase 2 enhanced features on a new feature branch.
+
+## Temporary Workarounds
+- None yet
+
+## Cross-Phase Dependencies
+- Phase 1 (OAuth) is independent — touches auth system only
+- Phase 2 (DnD) is independent — touches board editor UI only
+- Phase 3 (Widgets) depends on existing widget system from MVP
+- Phase 4 (Access Control) depends on existing permission system from MVP
+- Phase 5 (Integration) depends on all prior phases
+
+## Implementation Notes
+- Big Bang strategy: intermediate phases may not build. Phase 5 is the convergence phase.
+- OAuth uses `openid-client` (already installed in MVP dependencies)
+- DnD uses `svelte-dnd-action` (needs to be installed)
+- New widget types extend the existing Widget model's `type` and `config` JSON fields