Enhance annotations: save/download, fallback classes, photo mode icons

Add local annotation save fallback, PNG+txt download with drawn boxes,
shared classColors helper, photo mode icon toggles, and react-dropzone
/ react-icons dependencies.

src/features/annotations/classColors.ts

@@ -0,0 +1,24 @@
+const CLASS_COLORS = [
+  '#FF0000', '#00FF00', '#0000FF', '#FFFF00', '#FF00FF', '#00FFFF',
+  '#800000', '#008000', '#000080', '#808000', '#800080', '#008080',
+]
+
+export const FALLBACK_CLASS_NAMES = [
+  'Car', 'Person', 'Truck', 'Bicycle', 'Motorcycle', 'Bus',
+  'Animal', 'Tree', 'Building', 'Sign', 'Boat', 'Plane',
+]
+
+export function getClassColor(classNum: number): string {
+  const base = classNum % 20
+  return CLASS_COLORS[base % CLASS_COLORS.length]
+}
+
+export function getPhotoModeSuffix(classNum: number): string {
+  const mode = Math.floor(classNum / 20)
+  return mode === 1 ? ' (winter)' : mode === 2 ? ' (night)' : ''
+}
+
+export function getClassNameFallback(classNum: number): string {
+  const base = classNum % 20
+  return FALLBACK_CLASS_NAMES[base % FALLBACK_CLASS_NAMES.length] ?? `#${classNum}`
+}