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.

server/src/migrations/002_plot_beats.sql

@@ -0,0 +1,19 @@
+-- 002_plot_beats.sql — plot beats for the Plot board and Timeline (CHUNK #5).
+
+CREATE TABLE IF NOT EXISTS plot_beats (
+  id         TEXT NOT NULL,
+  project_id TEXT NOT NULL REFERENCES projects(id) ON DELETE CASCADE,
+  title      TEXT NOT NULL DEFAULT '',
+  body       TEXT NOT NULL DEFAULT '',
+  act        INTEGER NOT NULL DEFAULT 1,
+  type       TEXT NOT NULL DEFAULT 'note',
+  beat_order INTEGER NOT NULL DEFAULT 0,
+  status     TEXT NOT NULL DEFAULT 'draft',
+  chapter_id TEXT,
+  entity_ids TEXT NOT NULL DEFAULT '[]',
+  created_at TEXT NOT NULL,
+  updated_at TEXT NOT NULL,
+  PRIMARY KEY (project_id, id)
+);
+CREATE INDEX IF NOT EXISTS idx_plot_beats_project ON plot_beats(project_id);
+CREATE INDEX IF NOT EXISTS idx_plot_beats_act ON plot_beats(project_id, act, beat_order);