Ratchet Biome to a blocking CI gate

`npm run lint` is now a required CI step (removed continue-on-error). It fails on
lint ERRORS — Biome's full recommended set, which currently has zero violations
outside the grandfathered backlog — so new error-class issues are blocked.

Grandfathered the entire pre-existing backlog (exactly the 16 rules that were
firing = all 713 prior errors) to "warn" in biome.json — visible in output, but
non-blocking — pending dedicated passes:
- a11y (~632): JSX needs button types, key handlers, svg titles, semantic
  elements, valid anchors — a real accessibility initiative of its own.
- suspicious/noArrayIndexKey (50), correctness/useExhaustiveDependencies (11):
  large, judgment-heavy; auto-fixing hook deps risks render loops.
- A few idiomatic/benign ones left as warn rather than churn working code:
  noAssignInExpressions (the `(m[k] ||= []).push()` idiom),
  useIterableCallbackReturn (nested forEach arrows), useJsxKeyInIterable (static
  [id,label,<svg>] config tuples), noImplicitAnyLet (consistent with
  tsconfig noImplicitAny:false).

Net: 0 errors / 920 warnings / 145 infos; `biome check` and `npm test` green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: DaviRain-Su

.github/workflows/ci.yml

@@ -17,11 +17,11 @@ jobs:
           node-version-file: .node-version
           cache: npm
       - run: npm ci
-      # Biome lint/format is advisory for now (the codebase predates it). Surfaces
-      # findings without failing the build; ratchet to a blocking gate once the
-      # one-time format pass + correctness fixes land. See biome.json.
-      - name: Biome (advisory)
+      # Biome gate: fails the build on lint ERRORS (formatting + Biome's
+      # recommended rule set). The pre-existing JSX a11y / array-index-key /
+      # hook-deps backlog is grandfathered to warnings in biome.json (visible but
+      # non-blocking) pending a dedicated pass; new error-class issues are blocked.
+      - name: Biome
         run: npm run lint
-        continue-on-error: true
       - run: npm test
       - run: npm run build

biome.json

@@ -34,7 +34,27 @@
     "rules": {
       "recommended": true,
       "suspicious": {
-        "noExplicitAny": "off"
+        "noExplicitAny": "off",
+        "noArrayIndexKey": "warn",
+        "useIterableCallbackReturn": "warn",
+        "noAssignInExpressions": "warn",
+        "noImplicitAnyLet": "warn"
+      },
+      "correctness": {
+        "useExhaustiveDependencies": "warn",
+        "useJsxKeyInIterable": "warn"
+      },
+      "a11y": {
+        "noStaticElementInteractions": "warn",
+        "useButtonType": "warn",
+        "useKeyWithClickEvents": "warn",
+        "noSvgWithoutTitle": "warn",
+        "useValidAnchor": "warn",
+        "useSemanticElements": "warn",
+        "noAutofocus": "warn",
+        "useFocusableInteractive": "warn",
+        "useAriaPropsSupportedByRole": "warn",
+        "noNoninteractiveElementToInteractiveRole": "warn"
       }
     }
   },