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The SvelteKit app under /frontend has fully superseded the legacy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- Repoint docs/DESIGN-SYSTEM.md links; remove 5 superseded docs/plan/*.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  (superseded by /resources).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>

scripts/gen-token-docs.mjs

@@ -1,13 +1,16 @@
 #!/usr/bin/env node
 // Generates docs/TOKENS.md — a grouped reference of every design token defined
-// in static/css/base/variables.css. Run after changing tokens:
+// in frontend/src/lib/styles/base/variables.css. Run after changing tokens:
 //   node scripts/gen-token-docs.mjs
 import { readFileSync, writeFileSync } from 'node:fs';
 import { join, dirname } from 'node:path';
 import { fileURLToPath } from 'node:url';
 
 const repo = join(dirname(fileURLToPath(import.meta.url)), '..');
-const src = readFileSync(join(repo, 'static', 'css', 'base', 'variables.css'), 'utf8');
+const src = readFileSync(
+    join(repo, 'frontend', 'src', 'lib', 'styles', 'base', 'variables.css'),
+    'utf8'
+);
 
 const tokens = [];
 for (const m of src.matchAll(/^\s*(--[a-z0-9-]+)\s*:\s*([^;]+);/gim)) {
@@ -46,7 +49,7 @@ for (const [name, value] of tokens) {
 
 let md = `# Design tokens
 
-> Auto-generated from \`static/css/base/variables.css\` by \`scripts/gen-token-docs.mjs\`.
+> Auto-generated from \`frontend/src/lib/styles/base/variables.css\` by \`scripts/gen-token-docs.mjs\`.
 > Do not edit by hand — re-run the generator after changing tokens.
 
 **${tokens.length} tokens** across ${[...buckets].filter(([, v]) => v.length).length} groups.