Enhance timeline and movement flow UI; add fetched painting images.

Timeline: era and historical-event click-to-zoom, event markers from historical-events.ts. Movement flow: shared zoom/pan, viewport-fit layout, curved lineage streams, hover-to-reveal artist lifespans on stacked lanes. Docs updated. Includes batch-fetched paintings and regenerated thumbs.
Co-authored-by: Cursor <cursoragent@cursor.com>

client/src/data/historical-events.ts

@@ -0,0 +1,35 @@
+/** Major world events shown as markers on the timeline (frontend curation). */
+export interface HistoricalEvent {
+  id: string;
+  name: string;
+  startYear: number;
+  /** Omit for a single-year marker. */
+  endYear?: number;
+  /** Shorter label when the view is crowded. */
+  shortLabel?: string;
+}
+
+export const HISTORICAL_EVENTS: readonly HistoricalEvent[] = [
+  { id: 'fall-rome', name: 'Fall of Rome', startYear: 476 },
+  { id: 'black-death', name: 'Black Death', startYear: 1347 },
+  { id: 'printing-press', name: 'Printing press', startYear: 1450 },
+  { id: 'reformation', name: 'Protestant Reformation', startYear: 1517 },
+  { id: 'columbus', name: 'Columbus reaches the Americas', startYear: 1492 },
+  { id: 'american-revolution', name: 'American Revolution', startYear: 1776 },
+  { id: 'french-revolution', name: 'French Revolution', startYear: 1789 },
+  { id: 'waterloo', name: 'Battle of Waterloo', startYear: 1815 },
+  { id: 'revolutions-1848', name: 'Revolutions of 1848', startYear: 1848 },
+  { id: 'german-unification', name: 'Unification of Germany', startYear: 1871 },
+  { id: 'ww1', name: 'World War I', startYear: 1914, endYear: 1918, shortLabel: 'WWI' },
+  { id: 'russian-revolution', name: 'Russian Revolution', startYear: 1917 },
+  { id: 'ww2', name: 'World War II', startYear: 1939, endYear: 1945, shortLabel: 'WWII' },
+  { id: 'berlin-wall', name: 'Fall of Berlin Wall', startYear: 1989 },
+];
+
+export function eventEndYear(event: HistoricalEvent): number {
+  return event.endYear ?? event.startYear;
+}
+
+export function eventInView(event: HistoricalEvent, viewStart: number, viewEnd: number): boolean {
+  return eventEndYear(event) >= viewStart && event.startYear <= viewEnd;
+}