Add performance optimizations to the UI hot paths

[quantdale]

The whole UI is rebuilt inside update(), which egui runs on every repaint —
including every mouse move — so per-frame work scaled with the size of the
snippet library instead of with what is on screen. Measured on a 1000-snippet
library (release build, 2 KB bodies), one grid frame took ~65 ms; it now takes
~2.4 ms, and the search filter went from ~2 ms/frame to ~5 us/frame.

• Virtualize the card grid. card_grid() lays out only the rows intersecting
  the viewport (plus a row of overscan) and reserves the height of the skipped
  rows, so the scrollbar and every card position are unchanged. Card rects for
  drag-and-drop now come from grid_card_rect(), computed from the grid origin,
  so a drop onto a gap that was never rendered still lands in the right slot.
  Row widget ids are keyed on the row index (push_id) so they no longer shift
  as rows above the viewport are skipped.
• Cache per-snippet derived text (Derived): the lowercase title/body/category
  the search matches against and the collapsed card preview. These were
  recomputed from scratch on every frame, lowercasing every body in the library
  and re-collapsing every visible preview.
• Memoize the visible-card index list (FilterCache), keyed on the query, the
  category filter, and a library generation counter, reusing its allocation
  across frames. All snippet mutations now run through snippets_changed(),
  which is what keeps the caches consistent with snippets.
• Stop rebuilding egui::Visuals every frame; apply the theme at startup and
  when the selection changes. Add Theme::name() -> &'static str (Display
  defers to it) so naming a theme doesn't allocate.
• Drop per-frame clones: the category list handed to the editor, the save-error
  banner text, and the combo-box labels. Move the editor's body into the
  snippet on save instead of copying it.
• Stop collapsing a whole snippet body just to preview its first 220 chars, and
  sort the canonical category list once per batch instead of once per snippet at
  startup (Config::add_categories); skip the allocations in same_category
  for the ASCII names it is called with.

Tests cover the visible-row range, computed-vs-rendered card rects, the paint
work of a ten-times-larger library, and the memoized filter against a fresh
scan; docs record the new invariants.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01Fbmt6KmCj5NsuPVQLvnt8J