feat(photos): keyboard a11y + shift-click range selection

- Tiles are focusable (tabindex / role=button / aria-label) with a
  :focus-visible ring; Enter opens the lightbox (or toggles in selection
  mode), Space toggles selection.
- Shift-click extends the selection from the last anchor across the
  timeline; the range is tracked in the selection Set so it spans
  dematerialized (off-screen) groups, with visible tiles updated at once.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JW6ghFMDtnRYuYNzZhb47M

static/css/views/photos.css

@@ -113,6 +113,11 @@
     border-color: var(--color-border-medium);
 }
 
+.photo-tile:focus-visible {
+    outline: 2px solid var(--color-accent);
+    outline-offset: 2px;
+}
+
 .photo-tile:hover img {
     transform: scale(1.03);
 }