Show reference-data update time, location filter UI, and tile fixes

- Status gains reference_updated_at (sourced from reference.db's file
  mtime, so it needs no separate persisted state) and the sidebar's
  "Your library" panel shows it as a relative "Updated Xh ago" stamp
  that self-refreshes every minute.
- Add a Location field to the Library tab's filters panel, wired to the
  new LibraryFilter.location.
- Bump every font-size in the stylesheet up one step for a slightly
  larger, easier-to-read UI overall.
- Fix the library/deck tile view's +/- quantity overlay permanently
  covering the card art's own printed mana cost (top-right corner) —
  it now only fades in on hover/focus, like the other row actions.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/api.ts

@@ -46,6 +46,7 @@ export interface LibraryFilter {
   subtypes?: string[];
   keywords?: string[];
   in_deck?: number | null;
+  location?: string | null;
 }
 
 export interface Status {
@@ -53,6 +54,7 @@ export interface Status {
   reference_path: string;
   reference_ready: boolean;
   card_count: number;
+  reference_updated_at: number | null; // unix seconds
 }
 
 export interface LibraryStats {