fix(ci): make SonarQube actually analyse the frontend (#67)

SonarQube was analysing none of frontend/src. All 34 files were indexed and reported ncloc 0, so the quality gate had been measuring roughly a third of the codebase while appearing to cover it. The scanner log gives the cause: frontend/tsconfig.json sets "moduleResolution": "bundler", which is correct for Vite, and the TypeScript bundled with SonarQube 9.9 predates 5.0 and rejects it. Building the frontend program throws, every frontend file is dropped, and the scan still exits EXECUTION SUCCESS — which is why a green job hid this indefinitely.

Adds frontend/tsconfig.sonar.json, an analysis-only mirror that differs only in using "node", and points the scan at it via sonar.typescript.tsconfigPaths. The app's own tsconfig is deliberately untouched: "bundler" is right for the build, and changing it to satisfy an old analyser would let the tool dictate the build. The mirror cannot use `extends` — the old compiler validates the base file while reading it, so the error just moves to pointing at tsconfig.json.

Verified locally against a scratch project: 59/59 files analysed, no skips. Analysed lines go from 2,069 to 5,481, code smells from 2 to 14, security hotspots from 3 to 4, and technical debt from 21 to 85 minutes. The frontend had been hiding twelve code smells and a hotspot, which is part of why the React problems behind #60 and #62 had to be found by hand.

A standalone copy drifts, and drift here does not fail anything — it silently returns to skipping the frontend while reporting success. scripts/check-sonar-tsconfig.js compares the two and fails when they diverge in anything but moduleResolution, and runs before the scan so the scan is never what discovers it. Confirmed it catches drift by introducing some.

Moves scan settings into sonar-project.properties at the repo root so a local scan and the CI scan analyse the same thing, leaving only the host and token in secrets. Adds scripts/scan-local.sh, which runs the scanner in Docker because it needs Java 11+ and the dev machine has Java 8, and which defaults to a scratch project key: the server is Community edition with no branch analysis, so any scan overwrites the single main analysis of whichever key it is given.

Closes #67

.claude/project-context.md

@@ -236,6 +236,16 @@ The severity split is deliberate and is the whole design: every preset is downgr
 
 `@typescript-eslint/no-misused-promises` runs with `checksVoidReturn: { attributes: false }`, because `onClick={async () => ...}` is idiomatic React and safe when the handler catches its own errors — left at the default the rule flags every antd button in the admin screens.
 
+## SonarQube
+
+Server is **SonarQube 9.9.8 LTA, Community edition**, at the URL in `SONARQUBE_URL`. Scan settings live in `sonar-project.properties` at the repo root, not as inline `-D` args, so a local scan and the CI scan analyse the same thing; only the host and token come from Gitea secrets.
+
+**Community edition has no branch analysis.** Every scan overwrites the single `main` analysis of whatever project key it is given, so scanning a feature branch under the real key silently replaces CI's picture of main with your working tree. `scripts/scan-local.sh` therefore defaults to the scratch key `redefined-designs-local`; pass `redefined-designs` explicitly to publish for real. The scanner runs in Docker because it needs Java 11+ and the dev machine's Java is 8.
+
+**`frontend/tsconfig.sonar.json` is load-bearing, and its failure mode is silent.** SonarQube 9.9's bundled TypeScript predates 5.0 and rejects `"moduleResolution": "bundler"`, which `frontend/tsconfig.json` needs for Vite. Without the shim the frontend program fails to build, all 34 frontend files are skipped, and **the scan still exits `EXECUTION SUCCESS`** — the state #67 found, where the gate had been reporting on a third of the codebase while looking complete. The shim cannot use `extends`: the old compiler validates the base file while reading it, so the error fires before any override applies. `scripts/check-sonar-tsconfig.js` runs before the scan in CI and fails if the copy drifts from the real tsconfig in anything but `moduleResolution`. Delete the shim, the check, and the `sonar.typescript.tsconfigPaths` line together once the server is new enough to parse `bundler`.
+
+Take a green SonarQube job as weak evidence. It exits success on a partially-failed analysis, so the number worth checking after a scan is `ncloc` — if it drops sharply, something is being skipped.
+
 ## Testing
 
 - **Unit tests**: `cd backend && npm run test:unit` — no DB required