Local-only app: remove network, uploads, providers, backend, desktop

Phone is now a pure on-device recorder: no accounts, no servers, no
background uploads (INTERNET permission gone). File sync is the user's
own tooling; the library adopts externally added files.

Android:
- Delete provider package (Nextcloud, custom SHONAR, sync-folder,
  registry, auth, TOFU/TLS), sync stack (SyncWorker/Drain/Slots,
  MigrationRunner), provider/storage/folder UI, AI-via-server details.
- Home shows a fixed 'on this phone' library; details keep playback,
  rename, file info. Settings lose Network/provider/HTTP-logging.
- Library root is the stored folder or Music/Recordings; import is
  double-scan proof (mutex + unique filePath index, migration v3->v4
  dedupes by path) with regression tests.
- Drop okhttp/work/security-crypto/media deps; delete their tests.

Repo: backend/, desktop/, worker/, deploy/, shared/, server scripts
and docs removed; README rewritten; CI keeps the android job only.

.github/workflows/ci.yml

@@ -6,34 +6,6 @@ on:
   pull_request:
 
 jobs:
-  backend:
-    runs-on: ubuntu-latest
-    services:
-      postgres:
-        image: postgres:16-alpine
-        env:
-          POSTGRES_USER: shonar
-          POSTGRES_PASSWORD: shonar
-          POSTGRES_DB: shonar_test
-        ports: ["5432:5432"]
-        options: >-
-          --health-cmd "pg_isready -U shonar"
-          --health-interval 5s --health-timeout 3s --health-retries 10
-    defaults:
-      run:
-        working-directory: backend
-    steps:
-      - uses: actions/checkout@v4
-      - uses: astral-sh/setup-uv@v4
-      - uses: actions/setup-python@v5
-        with:
-          python-version: "3.11"
-      - run: uv venv .venv && uv pip install -e ".[dev]"
-      - run: .venv/bin/ruff check .
-      - run: .venv/bin/pytest
-        env:
-          SHONAR_TEST_DATABASE_URL: postgresql+asyncpg://shonar:shonar@localhost:5432/shonar_test
-
   android:
     runs-on: ubuntu-latest
     defaults: