feat(library): sort and badge by personal difficulty rating (#810)

* feat(library): sort and badge by personal difficulty rating

Adds sort=difficulty/difficulty-desc to the library API (correlated
subquery over song_user_meta.user_difficulty, unrated songs pushed to
the bottom either direction, same pattern as the existing mastery
sort) and surfaces the rating as a badge on library cards in both the
v2 grid/tree views and the v3 grid. The rating itself already existed
(song_user_meta) — this just makes it sortable and visible, so it's
no longer only readable in the per-song edit drawer.

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

* fix(library): escape difficulty badge, wire tree view, add changelog+tests

- Wrap song.user_difficulty in esc() at both badge call sites
  (static/app.js ~2082 and ~2283) for XSS-consistency with the
  sibling tuning badge, which already uses esc().
- server.py: query_artists (the classic tree view's data source, used
  by /api/library/artists) never batch-attached user_difficulty the
  way query_page does for the grid, so the tree-view difficulty badge
  added in 75673c3 was unreachable dead code (song.user_difficulty was
  always undefined there). Now attaches it via the existing
  user_meta_map() helper, same pattern as query_page.
- Add an [Unreleased] CHANGELOG.md entry for the difficulty sort +
  badge feature, matching the repo's existing entry format.
- Add tests/test_library_filters.py::test_difficulty_sort_pushes_unrated_to_bottom
  asserting unrated songs sort to the bottom in both sort=difficulty
  and sort=difficulty-desc directions, and
  ::test_tree_view_songs_carry_user_difficulty covering the
  query_artists fix above.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix(library): chunk user_meta_map + rebuild stale tailwind css

Address review-bot findings on the difficulty sort/badge:
- user_meta_map now chunks filenames into 400-row batches (like
  overrides_map) before the IN (...) query. query_artists (tree view)
  passes every song across up to 50 artists, which could push the
  placeholder count past SQLite's older variable limit; query_page's
  small pages are unaffected. (CodeRabbit: Stability & Availability)
- Rebuild static/tailwind.min.css: the ◆N difficulty badge introduced
  bg-blue-900/30 + text-blue-300, which were never compiled into the
  committed stylesheet, failing the tailwind-fresh CI gate.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
Co-authored-by: byrongamatos <xasiklas@gmail.com>

static/app.js

@@ -1110,6 +1110,7 @@ const _LIB_VIEW_VALUES = new Set(['grid', 'tree', 'folder']);
 const _LIB_SORT_VALUES = new Set([
     'artist', 'artist-desc', 'title', 'title-desc',
     'recent', 'year-desc', 'year', 'tuning',
+    'difficulty', 'difficulty-desc',
 ]);
 const _LIB_FORMAT_VALUES = new Set(['', 'sloppak', 'loose']);
 // Tree-view expand/collapse persistence. Three states per tree:
@@ -2078,6 +2079,7 @@ function renderGridCards(songs, containerId = 'lib-grid', mode = 'replace') {
                     ${(() => { const _nm = _getArrangementNamingMode(); return (song.arrangements || []).map(a => _arrangementBadgeHtml(a, _nm)).join(''); })()}
                     ${tuning ? `<span class="px-1.5 py-0.5 rounded ${tuning === 'E Standard' ? 'bg-green-900/30 text-green-400' : 'bg-yellow-900/30 text-yellow-400'}">${esc(tuning)}</span>` : ''}
                     ${song.has_lyrics ? `<span class="px-1.5 py-0.5 bg-purple-900/30 rounded text-purple-300">Lyrics</span>` : ''}
+                    ${song.user_difficulty != null ? `<span class="px-1.5 py-0.5 bg-blue-900/30 rounded text-blue-300" title="Your difficulty rating">◆${esc(song.user_difficulty)}</span>` : ''}
                     ${duration ? `<span class="text-gray-600">${duration}</span>` : ''}
                 </div>
                 ${retuneBtn}
@@ -2277,6 +2279,8 @@ async function renderTreeInto(containerId, countId, stats, letter, q, favoritesO
                     html += `<span class="px-1.5 py-0.5 rounded ${tuning === 'E Standard' ? 'bg-green-900/30 text-green-400' : 'bg-yellow-900/30 text-yellow-400'}">${esc(tuning)}</span>`;
                 if (song.has_lyrics)
                     html += `<span class="px-1.5 py-0.5 bg-purple-900/30 rounded text-purple-300">Lyrics</span>`;
+                if (song.user_difficulty != null)
+                    html += `<span class="px-1.5 py-0.5 bg-blue-900/30 rounded text-blue-300" title="Your difficulty rating">◆${esc(song.user_difficulty)}</span>`;
                 if (duration)
                     html += `<span class="text-gray-600 w-10 text-right">${duration}</span>`;
                 if (stdRetune)