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

frontend/tsconfig.sonar.json

@@ -0,0 +1,16 @@
+{
+  "//": "Analysis-only copy of tsconfig.json. SonarQube 9.9 bundles a TypeScript older than 5.0, which rejects moduleResolution \"bundler\" outright — the program fails to build and every frontend file is skipped while the scan still reports success. It cannot `extends` tsconfig.json either: the old compiler validates the base file as it reads it, so the error fires before any override applies. Keep this in step with tsconfig.json; scripts/check-sonar-tsconfig.js fails the CI scan if they drift. Delete it once the server is new enough to parse \"bundler\".",
+  "compilerOptions": {
+    "target": "ES2020",
+    "useDefineForClassFields": true,
+    "lib": ["ES2020", "DOM", "DOM.Iterable"],
+    "module": "ESNext",
+    "moduleResolution": "node",
+    "jsx": "react-jsx",
+    "strict": true,
+    "skipLibCheck": true,
+    "esModuleInterop": true,
+    "noEmit": true
+  },
+  "include": ["src"]
+}