Rename .scss → .css and remove sass dependency (Phase 9-10)

Rename all 93 .scss files to .css, inline remaining Sass-only
syntax (// comments → /* */, arithmetic → pre-calculated values,
local $vars → hardcoded), update JS entry point imports, remove
sass-loader from webpack, remove sass/sass-loader/stylelint-config-
standard-scss packages, and clean up extraction scripts.

Convert mastodon flavour SCSS to CSS custom properties (Phase 7)

Replace $var with var(--var) across all 23 mastodon/*.scss files + mailer.scss.
Inline avatar/search mixins, unroll @each/@for, hardcode hex-color() in data URIs.
Merge pre-calculated properties into variables.scss :root blocks for all 3 themes.
Fix theme import order, delete _mixins.scss and temporary properties files.

Fix merged PR bugs and rewrite reduced-motion to pure CSS

- PR #64: fix typo "ad" → "add" in bookmark folder empty message
- PR #64: fix reducer using wrong key ('folder'/'title' → 'name')
- PR #84: implement confirm_unfollow using local_settings instead of
  server-side unfollowModal global
- PR #104: replace JS body-class approach with native CSS
  @media (prefers-reduced-motion) queries across all SCSS files
- Wide view: columns fill available space, content centered at 600px

[build] upgrade to Prettier 3.6.2 (#86)

Upgrades Prettier to 3.6.2 and pins it there. Prettier [recommends pinning the version](https://prettier.io/docs/install#summary):

> Install an exact version of Prettier locally in your project. This makes sure that everyone in the project gets the exact same version of Prettier. Even a patch release of Prettier can result in slightly different formatting, so you wouldn’t want different team members using different versions and formatting each other’s changes back and forth.

---

Should probably merge after #85.

Co-authored-by: tobi <kipvandenbos@noreply.codeberg.org>
Reviewed-on: https://codeberg.org/superseriousbusiness/masto-fe-standalone/pulls/86
Co-authored-by: Zoë Bijl <code@moiety.me>
Co-committed-by: Zoë Bijl <code@moiety.me>

Merge remote-tracking branch 'upstream' into glitch

Merge pull request #2434 from ClearlyClaire/glitch-soc/merge-upstream

Merge upstream changes up to 640421f661ee4d7e76a2aab607e7b15687940b6f

Merge commit '37bbd3c1064380a756b4e821655d7697105f99f5' into glitch-soc/merge-upstream

Merge commit '5a437edc7f22d5797e5af29d9b450a03eccec304' into glitch-soc/merge-upstream

Merge commit 'e6b903b21e0682c1156ffec583ac2836fda7d657' into glitch-soc/merge-upstream

Fix double scroll bars in some columns in advanced interface (#27187)

Keep version string displayed without breakpoints in UI (#26986)

Make notification respect reduce-motion (#27178)

Merge pull request #2426 from ClearlyClaire/glitch-soc/merge-upstream

Merge upstream changes up to 57f592fed50747f3c97718a2761e17bafe6c8698

Merge commit '57f592fed50747f3c97718a2761e17bafe6c8698' into glitch-soc/merge-upstream

Fix line wrapping of language selection button with long locale codes (#27100)

Merge pull request #2406 from ClearlyClaire/glitch-soc/merge-upstream

Merge upstream changes up to 6273416292090b2f6bfda33f070cea325a1759df

Merge commit '6273416292090b2f6bfda33f070cea325a1759df' into glitch-soc/merge-upstream

Conflicts:
- `.github/workflows/build-nightly.yml`:
  Upstream modified this file close to a line modified by glitch-soc to
  disable the ARMv64 builder (as glitch-soc doesn't have one).
  Ported upstream's changes.
- `.github/workflows/build-releases.yml`:
  Upstream modified this file close to lines changed by glitch-soc to
  change the Docker image repositories used (only ghcr, and based on the
  organization's name).
  Ported upstream's changes.

Merge commit '97e4011c3ce31c6d492d5f103e95a35b6ebdc9bd' into glitch-soc/merge-upstream

Conflicts:
- `config/initializers/content_security_policy.rb`:
  Upstream added some code to add the Identity Provider's sign-in endpoint to
  the `form-action` Content Security Policy directive but our version of the
  file is pretty different.
  Ported the change.

Merge commit 'b1d89071384ef028c97a3d28cf8cf6bc0ca6c4ce' into glitch-soc/merge-upstream

Merge commit '398635c0c4987ec44d937e98431ff5dee331ea94' into glitch-soc/merge-upstream