Card preview modal, library quantity steppers, and UI polish

- Fix keyboard-nav selection highlight (search results, deck list): the
  Discord-styling rewrite dropped the --panel-2 CSS variable it relied
  on, so ↑/↓ moved selection but rendered no visible highlight.
- Add click-to-preview: clicking a card thumbnail/tile now opens a
  larger centered modal (list and tile views), with a flip button for
  double-faced cards' back face. Hover preview is unchanged and clears
  when a click-preview opens.
- Add a QtyStepper component (rounded [− N +] pill, with an `overlay`
  variant for tile corners) and use it consistently for owned-copy
  counts on Add Cards, and the Library list and tile views.
- Add matching +/- steppers to the Library views; decrementing to 0
  dims the row for a 5s grace period (still fully interactive) before
  it drops out, so an accidental removal can be undone with +.
- Topbar: fill remaining width with the library path (no more premature
  truncation), add icons to Open/New Library and Download/Update card
  data, and drop the trailing "…" from the library buttons.

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

src/api.ts

@@ -11,6 +11,7 @@ export interface CardRef {
   color_identity: number; // WUBRG bitmask
   type_line: string;
   image_uri: string; // Scryfall "normal" image URL, or "" if unavailable
+  back_image_uri: string; // back face for transform/MDFC/reversible cards, else ""
 }
 
 export interface LibraryCard extends CardRef {