Merge commit '28e5c3bb5116d012a1f58b4cbfe1a5fb302ac227' into glitch-soc/merge-upstream

Conflicts:
- `app/helpers/theme_helper.rb`:
  Upstream added fallback behavior for when the admin-set theme does not exist.
  Glitch-soc has a different theming system, but already had similar fallback,
  so not changing glitch-soc code.
- `spec/helpers/theme_helper_spec.rb`:
  Upstream adapted existing tests for the new fallback behavior, and added some
  tests accordingly.
  Glitch-soc has a different theming system, adapted the new tests accordingly.

package.json

@@ -1,7 +1,7 @@
 {
   "name": "@mastodon/mastodon",
   "license": "AGPL-3.0-or-later",
-  "packageManager": "yarn@4.13.0",
+  "packageManager": "yarn@4.14.1",
   "engines": {
     "node": ">=20"
   },