docs: add coverage thresholds, ./gradlew coverage, and LSP warning discipline

AGENTS.md:
  - Add "./gradlew coverage" to All-in-one quick-start section
  - Add "npm run test:coverage" to Frontend commands
  - Add Coverage section: command, threshold table (70% lines, 60%
    branches, 70% functions), HTML report paths for both layers
  - Note that coverage is enforced during ./gradlew check

CODING_GUIDELINES.md:
  - Section 1 (General Principles): add "Treat warnings as mistakes"
    rule — LSP diagnostics, compiler warnings, and lint warnings are
    bugs that must be fixed before commit
  - Known false positives (Lombok, getActivePinia) must be suppressed
    explicitly at the narrowest scope with a comment explaining why
  - Uncommented suppressions are treated as errors
  - Section 7 (Testing): add Coverage subsection with thresholds table,
    command reference, report paths, and enforcement rule (PRs must
    maintain or improve coverage)

AGENTS.md

@@ -37,7 +37,8 @@ docker compose up -d    # starts postgres, backend, frontend
 ### All-in-one
 
 ```bash
-./gradlew check         # frontend lint → frontend test → backend test → integration test
+./gradlew check         # frontend lint → frontend test → backend test → coverage verification
+./gradlew coverage      # backend + frontend tests with coverage reports
 ./gradlew up            # docker compose up -d
 ./gradlew down          # docker compose down
 ./gradlew reset         # docker compose down -v && docker compose up -d (full DB reset)
@@ -52,6 +53,7 @@ npm run dev          # dev server on :3000 with HMR
 npm run build        # production build
 npm run lint         # ESLint
 npm run test         # vitest
+npm run test:coverage # vitest with coverage (HTML at frontend/coverage/)
 ```
 
 ### Backend (Spring Boot 4 + Java 21)
@@ -228,6 +230,26 @@ the same PR — never merge code without corresponding tests.
 
 ---
 
+## Coverage
+
+```bash
+./gradlew coverage    # backend + frontend tests with coverage
+```
+
+Coverage thresholds are enforced during `./gradlew check`. PRs must maintain
+or improve coverage.
+
+| Layer    | Lines | Branches | Functions |
+|----------|-------|----------|-----------|
+| Backend  | 70%   | 60%      | —         |
+| Frontend | 70%   | 60%      | 70%       |
+
+HTML reports:
+- Backend: `backend/build/reports/jacoco/index.html`
+- Frontend: `frontend/coverage/index.html`
+
+---
+
 ## External References
 
 For detailed conventions, load `@CODING_GUIDELINES.md`.

CODING_GUIDELINES.md

@@ -15,6 +15,15 @@ Conventions and standards for the BilHej codebase. These exist to keep the proje
 - **No commented-out code.** Delete it. Git history keeps it if needed.
 - **Keep functions small.** A function should do one thing. If it's over 30 lines, it probably does too much.
 - **No magic numbers.** Use named constants or enums.
+- **Treat warnings as mistakes.** LSP diagnostics, compiler warnings, and lint
+  warnings are bugs. Never commit code that produces them. If a warning is a
+  known false positive (e.g. Lombok `@RequiredArgsConstructor` triggering
+  "uninitialized final field"), suppress it explicitly at the narrowest scope
+  with a comment explaining why:
+  - Java: `@SuppressWarnings("...") // Lombok generates constructor`
+  - TypeScript: `// @ts-expect-error — pinia getActivePinia returns null in test context`
+  Uncommented suppressions are indistinguishable from ignoring a real problem
+  and are treated as errors.
 
 ---
 
@@ -303,6 +312,24 @@ the same PR — never merge code without corresponding tests.
   raw SQL in test code. Tests interact with the database the same way
   production code does: through the ORM.
 
+### Coverage
+
+```bash
+./gradlew coverage    # backend + frontend tests with coverage
+```
+
+Coverage is enforced via `./gradlew check`. Thresholds:
+
+| Layer    | Lines | Branches | Functions |
+|----------|-------|----------|-----------|
+| Backend  | 70%   | 60%      | —         |
+| Frontend | 70%   | 60%      | 70%       |
+
+- Backend: JaCoCo (`backend/build/reports/jacoco/index.html`).
+- Frontend: Vitest v8 provider (`frontend/coverage/index.html`).
+- PRs must maintain or improve coverage levels. If a new feature changes
+  coverage, update the test suite — never lower thresholds without discussion.
+
 ---
 
 ## 8. Linting & Formatting