refactor: remove voting system, add author colors + pin numbering

- Remove votes backend (route, db functions, server mount)
- Remove votes frontend (store, socket, PinOverlay badges, FeedbackPanel UI)
- Add authorColors utility (8-color palette, deterministic hash)
- Add getPinNumber() to annotationStore
- Fix JWT to include username/display_name
- Add resolveAuthorName DB fallback for old tokens

frontend/src/utils/authorColors.ts

@@ -0,0 +1,35 @@
+const AUTHOR_COLORS = [
+  '#4a9eff', // blue
+  '#f97316', // orange
+  '#22c55e', // green
+  '#a855f7', // purple
+  '#ef4444', // red
+  '#06b6d4', // cyan
+  '#eab308', // yellow
+  '#ec4899', // pink
+];
+
+const AUTHOR_COLORS_HEX = [
+  0x4a9eff, 0xf97316, 0x22c55e, 0xa855f7,
+  0xef4444, 0x06b6d4, 0xeab308, 0xec4899,
+];
+
+function hashCode(str: string): number {
+  let hash = 0;
+  for (let i = 0; i < str.length; i++) {
+    hash = ((hash << 5) - hash + str.charCodeAt(i)) | 0;
+  }
+  return Math.abs(hash);
+}
+
+export function getAuthorColor(userId: string): string {
+  return AUTHOR_COLORS[hashCode(userId) % AUTHOR_COLORS.length];
+}
+
+export function getAuthorColorHex(userId: string): number {
+  return AUTHOR_COLORS_HEX[hashCode(userId) % AUTHOR_COLORS_HEX.length];
+}
+
+export function getAuthorInitial(name: string): string {
+  return (name || '?').charAt(0).toUpperCase();
+}