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/ui/src/AppShell.css

@@ -298,6 +298,8 @@ body {
 .ps-btn-lg { padding: 10px 20px; font-size: 15px; }
 .ps-btn-primary { background: var(--ps-accent); color: var(--ps-accent-contrast); }
 .ps-btn-primary:hover:not(:disabled) { filter: brightness(1.08); }
+.ps-btn-accent { background: var(--ps-accent); color: var(--ps-accent-contrast); border-color: var(--ps-accent); }
+.ps-btn-accent:hover:not(:disabled) { filter: brightness(1.08); }
 .ps-btn-secondary { background: var(--ps-surface-2); border-color: var(--ps-border); color: var(--ps-text); }
 .ps-btn-secondary:hover:not(:disabled) { background: var(--ps-border); }
 .ps-btn-ghost { background: transparent; color: var(--ps-text); }

packages/ui/src/components/Badge.tsx

@@ -3,8 +3,9 @@ export type BadgeVariant = "neutral" | "accent" | "success" | "danger" | "warnin
 export interface BadgeProps {
   children: string;
   variant?: BadgeVariant;
+  title?: string;
 }
 
-export function Badge({ children, variant = "neutral" }: BadgeProps) {
-  return <span className={`ps-badge ps-badge-${variant}`}>{children}</span>;
+export function Badge({ children, variant = "neutral", title }: BadgeProps) {
+  return <span className={`ps-badge ps-badge-${variant}`} title={title}>{children}</span>;
 }

packages/ui/src/components/Button.tsx

@@ -1,6 +1,6 @@
 import { type ButtonHTMLAttributes } from "react";
 
-export type ButtonVariant = "primary" | "secondary" | "ghost" | "danger";
+export type ButtonVariant = "primary" | "secondary" | "ghost" | "danger" | "accent";
 export type ButtonSize = "sm" | "md" | "lg";
 
 export interface ButtonProps extends ButtonHTMLAttributes<HTMLButtonElement> {