Separate tray app and core. Add rest API, Web UI.

.github/pull_request_template.md

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+# Pull Request
+
+## Description
+Brief description of the changes in this PR.
+
+## Type of Change
+- [ ] Bug fix (non-breaking change that fixes an issue)
+- [ ] New feature (non-breaking change that adds functionality)  
+- [ ] Breaking change (fix or feature that causes existing functionality to change)
+- [ ] Documentation update
+- [ ] Refactoring (code change that neither fixes a bug nor adds a feature)
+- [ ] Hotfix (critical production fix)
+
+## Testing
+- [ ] I have tested these changes locally
+- [ ] I have added/updated tests that prove my fix is effective or my feature works
+- [ ] All existing tests pass
+
+## Checklist
+- [ ] My code follows the project's style guidelines
+- [ ] I have performed a self-review of my code
+- [ ] I have commented my code, particularly in hard-to-understand areas
+- [ ] I have made corresponding changes to the documentation
+- [ ] My changes generate no new warnings
+
+## Hotfix Backport Reminder
+⚠️ **If this is a hotfix applied to `main`**: 
+- [ ] I have backported this fix to the `next` branch
+- [ ] OR I will create a separate PR to backport to `next` immediately after this merges
+
+**Hotfix backporting is critical** - see [docs/releasing.md](docs/releasing.md) for the complete hotfix process.
+
+## Additional Notes
+Any additional information, screenshots, or context about the changes.

.github/workflows/frontend.yml

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+name: Frontend CI
+
+on:
+  push:
+    branches: [ main, next ]
+    paths:
+      - 'frontend/**'
+      - '.github/workflows/frontend.yml'
+  pull_request:
+    branches: [ main, next ]
+    paths:
+      - 'frontend/**'
+      - '.github/workflows/frontend.yml'
+
+jobs:
+  frontend-test:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Setup Node.js
+      uses: actions/setup-node@v4
+      with:
+        node-version: '20'
+        cache: 'npm'
+        cache-dependency-path: frontend/package-lock.json
+
+    - name: Install dependencies
+      run: cd frontend && npm ci
+
+    - name: Type check
+      run: cd frontend && npm run type-check
+
+    - name: Lint
+      run: cd frontend && npm run lint
+
+    - name: Run tests
+      run: cd frontend && npm run test
+
+    - name: Build frontend
+      run: cd frontend && npm run build
+
+    - name: Upload build artifacts
+      uses: actions/upload-artifact@v4
+      with:
+        name: frontend-dist
+        path: frontend/dist/
+        retention-days: 7