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/views/DeckView.tsx

@@ -269,7 +269,7 @@ export function DeckView() {
           {decks.map((d) => (
             <li
               key={d.id}
-              style={d.id === selected ? { background: "var(--panel-2)" } : undefined}
+              style={d.id === selected ? { background: "var(--accent-subtle)" } : undefined}
               onClick={() => setSelected(d.id)}
             >
               <span className="card-name">{d.name}</span>