feat(plot): Chunk #5 story whiteboard, timeline & plot beats

Story whiteboard built on React Flow v11: free-form canvas of typed
nodes (character, world, chapter, note, image, group) with draggable
leads-to/contrasts/causes edges, cross-link selectors (chapter/entity),
linked-node badges, and an "Add beats to board" import from the plot.

Plot beats feature set:
- Status/locking (draft/revised/locked) with a progress bar
- Beat templates (three-act, save-the-cat, seven-point, custom)
- Timeline route with search/chapter/character filters
- Entity cross-links (entityIds) on beats and board nodes
- getBoard added to the StorageAdapter contract (local + server) and
  REST endpoints; loadBoard now restores persisted board state

Storage & server:
- PlotBeat type (status, entityIds) across core types, Dexie schema,
  and server mapping/routes/migration (002_plot_beats.sql)
- SQLite-safe column names (beat_order, entity_ids)
- Server seed data for plot beats

UI: accent Button variant, Badge variants, plot/timeline/whiteboard
stylesheets. Tests: templates module, plot-beat store round-trip,
board getBoard. typecheck, lint, test, and build all green.

packages/core/src/autosave.test.ts

@@ -40,6 +40,14 @@ function mockAdapter(): StorageAdapter & { saved: DocumentRef[] } {
       return [];
     },
     async saveBoard() {},
+    async getBoard() {
+      return null;
+    },
+    async savePlotBeat() {},
+    async listPlotBeats() {
+      return [];
+    },
+    async deletePlotBeat() {},
     async saveAISession() {},
     async saveDocument(doc: DocumentRef) {
       saved.push(doc);