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The SvelteKit app under /frontend has fully superseded the legacy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- Repoint docs/DESIGN-SYSTEM.md links; remove 5 superseded docs/plan/*.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  (superseded by /resources).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

docs/DESIGN-SYSTEM.md

@@ -9,7 +9,7 @@ and [UIUX-ROADMAP.md](UIUX-ROADMAP.md) (the work plan).
 ## 1. Design tokens
 
 Everything visual routes through a token in
-[`static/css/base/variables.css`](../static/css/base/variables.css). Six scales,
+[`static/css/base/variables.css`](../frontend/src/lib/styles/base/variables.css). Six scales,
 plus a curated color system. **Never hand-write a raw px/hex for these** — consume
 a token so the next change happens in one place.
 
@@ -40,7 +40,7 @@ These are enforced/aided by CI scripts and must hold for every new surface.
 - **Keyboard.** Everything interactive is reachable and operable by keyboard.
   Use `<button>`/`<a>` (not clickable `<div>`s). Nav exposes `aria-current="page"`.
 - **Focus.** A global `:focus-visible` ring lives in
-  [`base/a11y.css`](../static/css/base/a11y.css). Never `outline: none` without a
+  [`base/a11y.css`](../frontend/src/lib/styles/base/a11y.css). Never `outline: none` without a
   paired `:focus-visible` style. Mouse focus stays ring-free; keyboard focus never.
 - **Contrast.** Every text/background pair clears 4.5:1 in **light AND dark** —
   `node scripts/check-contrast.mjs` fails the build otherwise.
@@ -52,7 +52,7 @@ These are enforced/aided by CI scripts and must hold for every new surface.
   globally. Also honor `prefers-contrast` and `forced-colors`.
 - **Dialogs.** `role="dialog"` + `aria-modal` + `aria-labelledby`, focus trapped
   while open, focus restored to the trigger on close (see
-  [`modal.js`](../static/js/components/modal.js)).
+  [`modal.js`](../frontend/src/lib/components/Modal.svelte)).
 - **Icon-only buttons.** Always an `aria-label`; decorative icons get
   `aria-hidden="true"`.
 - **Touch targets.** ≥44×44px on phones.
@@ -61,7 +61,7 @@ These are enforced/aided by CI scripts and must hold for every new surface.
 
 ## 3. Brand
 
-- **Mark.** The cloud glyph ([`logo-plain.svg`](../static/logo/logo-plain.svg)).
+- **Mark.** The cloud glyph ([`logo-plain.svg`](../frontend/static/logo/logo-plain.svg)).
   Always rendered from the real SVG (never a stock `fa-cloud`). On surfaces it
   sits in an accent-gradient tile via the `.brand-mark` component.
 - **Wordmark.** "OxiCloud", weight 700, tight tracking. The accent-coloured
@@ -74,8 +74,8 @@ These are enforced/aided by CI scripts and must hold for every new surface.
   the mark; don't render the wordmark below ~16px.
 - **Don't:** recolor the mark, stretch it, place it on a low-contrast background,
   or introduce a second "primary" hue.
-- **Maskable / OG.** [`logo-maskable.svg`](../static/logo/logo-maskable.svg) (PWA,
-  safe-zone) and [`og-image.svg`](../static/logo/og-image.svg) (social). Export
+- **Maskable / OG.** [`logo-maskable.svg`](../frontend/static/logo/logo-maskable.svg) (PWA,
+  safe-zone) and [`og-image.svg`](../frontend/static/logo/og-image.svg) (social). Export
   both to PNG for full platform support (see roadmap).
 
 ---