Add saved drill presets to opening selection

Introduce a saved-drill preset system in the opening/endgame selection
modal, persisted to localStorage and keyed by a per-selection signature
(opening, variation, rating, color, move count, endgame traits).

- New src/lib/savedDrills.ts: read/write/upsert presets, signature and
  default-name helpers, with normalization and de-duplication.
- Saved Drills category in the browse panel, filtered by the active
  category and search term, with select/remove.
- Save acts on the queue when it has drills, otherwise on the drill
  currently configured in the preview, so tweaking a loaded preset's
  settings (e.g. rating) saves as a distinct drill rather than a no-op.
- Selection titles show the variation as the primary label and the
  opening family as the secondary label, in both the saved-drills list
  and the queue.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: ashtonanderson