chore(frontend): remove the legacy vanilla-JS frontend and its tooling

The SvelteKit app under /frontend has fully superseded the legacy
vanilla-JS/CSS frontend in /static, which was only ever served by a
debug `cargo run` / `PROFILE=dev` and never shipped to production.
Remove it together with the whole subsystem that existed only to
support it (~54k lines).

Frontend & assets:
- Delete /static (js/, css/, *.html, sw.js, basemaps/, locales symlink).
- Relocate the brand/PWA assets (logo/, favicon.ico, manifest.webmanifest)
  to frontend/static/ so they ship with the SPA. This also fixes the
  favicon, which app.html referenced but was missing from the prod bundle.
- Migrate the Nextcloud login-flow redirects from /nextcloud-error.html
  to the SvelteKit /nextcloud/error route.

Web layer:
- Simplify resolve_static_path: drop the PROFILE=dev branch; always prefer
  the Vite static-dist/ build, fall back to the configured path.
- Resolve i18n locales from the served SPA dir with a frontend/static
  fallback so `just dev` works without a prior build.

Build:
- Prune build.rs from 1262 to ~70 lines (git metadata only); the Rust asset
  pipeline and the OXICLOUD_RUST_ASSETS rollback flag are gone.
- Drop the now-unused build-dependencies (oxc_*, lightningcss).
- Remove the COPY static lines from the Dockerfile (cacher + builder).

Tooling & docs:
- Delete biome.json, jsconfig.json, tools/check-*.py, identifier.sh.
- Remove the legacy front-* justfile recipes; repoint the design-system
  scripts (locales, dead-tokens, brand-drift, token-docs) at the frontend,
  and drop check-contrast/check-headings (coupled to the old token
  taxonomy / multi-page HTML).
- Repoint docs/DESIGN-SYSTEM.md links; remove 5 superseded docs/plan/*.

Backend dead code:
- Remove the dead `folder_repo` field from FileBlobWriteRepository.
- Remove the deprecated GET /api/folders/{id}/listing endpoint
  (superseded by /resources).

Verified: cargo clippy (all-features/all-targets) clean, cargo test
--workspace 448 passed, cargo fmt clean.

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

justfile

@@ -100,36 +100,10 @@ db:
 db-down:
     docker compose down
 
-# start OxiCloud with static assets in no-cache mode
-front-dev:
-    PROFILE=dev cargo run
-
-# front: check all (linter, format, type, icons, translations...)
-front-check: front-fmt front-lint front-type front-rules front-check-icons front-check-i18n frontend-check
-
-# kept for compatibility
+# Frontend design-system / a11y guardrails (pure Node, no deps). The entry
+# point the UI/UX workflow uses; delegates to `front-design`.
 frontend-check: front-design
 
-front-fmt:
-    biome format static/
-
-front-lint:
-    biome lint static/
-
-# test types (JSDOC), using typescript
-front-type:
-    tsc -p jsconfig.json --noEmit
-
-# check CSS rules
-front-rules:
-    stylelint static/css/
-
-front-check-icons:
-    tools/check-icons.py --check-only
-
-front-check-i18n:
-    tools/check-missing-translations.py --check-only
-
 # end-to-end Playwright tests
 front-test:
     cd tests/e2e && npm test
@@ -138,14 +112,11 @@ front-test:
 front-test-update-snapshot:
     cd tests/e2e && npm test -- --update-snapshots=all
 
-# Frontend design-system / a11y guardrails — pure Node, no extra deps.
-# Single gate: WCAG contrast, heading order, locale completeness, dead tokens,
-# brand-mark drift.
-# (Also run `stylelint static/css/**/*.css`, `biome check`, `tsc -p jsconfig.json
-#  --noEmit` once node_modules is installed — those need devDependencies.)
+# Frontend design-system guardrails — pure Node, no extra deps, run against the
+# SvelteKit frontend (frontend/). Locale completeness, dead-token report, and
+# brand-mark drift. For the full svelte-check/eslint/stylelint/prettier gate use
+# `just fe-check` (needs the frontend devDependencies installed).
 front-design:
-    node scripts/check-contrast.mjs
-    node scripts/check-headings.mjs
     node scripts/check-locales.mjs
     node scripts/check-dead-tokens.mjs
     node scripts/check-brand-drift.mjs
@@ -157,9 +128,8 @@ api-test:
     bash tests/webdav/run.sh
 
 # ---------------------------------------------------------------------------
-# New SvelteKit frontend (frontend/). The original vanilla frontend (static/)
-# and its `front-*` recipes remain until the Phase 5 cutover; these `fe-*`
-# recipes drive the rewrite in the meantime.
+# SvelteKit frontend (frontend/) — the only frontend. These `fe-*` recipes
+# drive its dev server, build, lint and tests.
 # ---------------------------------------------------------------------------
 
 # install frontend dependencies