feat(exif): extract capture date and surface as sortable taken_at field

Adds EXIF date extraction during indexing so photos can be sorted by when
they were actually taken, not just when the file was modified on disk.

- Add `taken_at` (nullable TEXT, ISO 8601) column to the images table via
  ensure_column migration and a new idx_images_taken_at index
- Populate taken_at in build_record via extract_exif_date, which tries
  DateTimeOriginal → DateTimeDigitized → DateTime tags and rejects all-zero
  sentinel dates written by uninitialised cameras
- upsert_image uses taken_at = excluded.taken_at so a re-indexed file gets
  fresh EXIF; stale dates from replaced files are not preserved
- Add taken_asc / taken_desc sort using COALESCE(taken_at, created_at) so
  images without EXIF gracefully fall back to their creation date
- Surface as "Taken: newest / oldest" in the Toolbar sort dropdown
- Shift map_image_row column indices after the new taken_at (index 10)

src/store.ts

@@ -37,6 +37,7 @@ export interface ImageRecord {
   file_size: number;
   created_at: string | null;
   modified_at: string | null;
+  taken_at: string | null;
   mime_type: string;
   media_kind: MediaKind;
   duration_ms: number | null;
@@ -214,7 +215,9 @@ export type SortOrder =
   | "rating_desc"
   | "rating_asc"
   | "duration_desc"
-  | "duration_asc";
+  | "duration_asc"
+  | "taken_desc"
+  | "taken_asc";
 
 interface GalleryState {
   folders: Folder[];
@@ -494,6 +497,10 @@ function compareImages(a: ImageRecord, b: ImageRecord, sort: SortOrder): number
       return compareNullableNumber(a.duration_ms, b.duration_ms);
     case "duration_desc":
       return compareNullableNumber(b.duration_ms, a.duration_ms);
+    case "taken_asc":
+      return compareNullableDate(a.taken_at ?? a.created_at, b.taken_at ?? b.created_at);
+    case "taken_desc":
+      return compareNullableDate(b.taken_at ?? b.created_at, a.taken_at ?? a.created_at);
     default:
       return compareNullableDate(b.modified_at, a.modified_at);
   }