feat(themes): Theme Store install counts, downloads & sort (#1036)

* feat(themes): install counts, downloads, popularity & sort in the Theme Store

Each registry theme now carries an install count and a last-changed date. Store
rows show them in a dedicated meta panel (author, popularity bar, total
downloads, last changed), plus a sort dropdown (most popular / newest / name),
zebra-striped rows, a numbered pager, and a note that the stats refresh daily.
Adds a shared formatRelativeTime helper (formatLastSeen now delegates to it).

* test(themes): cover sort modes, pager jump and relative-time formatting

* fix(build): gate nvidia_quirk_active to Linux

Its only caller is the Linux branch of theme_animation_risk, so on Windows and
macOS the function was dead code and tripped clippy's -D warnings.

* docs: CHANGELOG + credits for theme store download stats & sort (#1036)

src/locales/en/settings.ts

@@ -396,6 +396,7 @@ export const settings = {
   themeStoreSubmitText: 'Made your own theme? Share it with the community — more info in the themes repository.',
   themeStoreSubmitLink: 'Open the themes repository',
   themeStoreNetworkNotice: 'The Theme Store loads its catalogue and previews from external services (the jsDelivr CDN and GitHub). No personal data is sent.',
+  themeStoreStatsNotice: 'Download and last-changed stats refresh once a day (around 04:17 UTC).',
   themeStoreSearchPlaceholder: 'Search themes…',
   themeStoreFilterMode: 'Filter by mode',
   themeStoreModeAll: 'All',
@@ -409,7 +410,13 @@ export const settings = {
   themeStoreEmpty: 'No themes match your filters.',
   themeStoreActive: 'Active',
   themeStoreEnlarge: 'Enlarge preview',
-  themeStoreByAuthor: 'by {{author}}',
+  themeStoreSortPopular: 'Most popular',
+  themeStoreSortNewest: 'Newest',
+  themeStoreSortName: 'Alphabetical',
+  themeStoreAuthor: 'Author',
+  themeStorePopularity: 'Popularity',
+  themeStoreDownloads: 'Total downloads',
+  themeStoreLastChanged: 'Last changed',
   themeMigrationNoticeTitle: 'Theme moved to the Store',
   themeMigrationNoticeBody: '{{themes}} is no longer bundled with the app. You can reinstall it from the Theme Store (Settings → Themes), or pick another theme.',
   themeMigrationNoticeOpen: 'Open Theme Store',