feat(ci): add ESLint to both workspaces with a defect-only CI gate (#60)

TypeScript's strict mode checks types and nothing else, so nothing enforced the React hook rules, the SonarJS rules, or unhandled-promise detection. Adds a flat config per workspace, a lint script in each, and a lint job in tests.yml.

The rule selection is the substance of this change and is measured rather than guessed. A full-strength config reports 435 violations across 50 files, but 325 of those are the no-unsafe-* family from recommendedTypeChecked, every one downstream of pool.query() returning any rows and untyped fetch responses. Typing those boundaries is the whole of #65, so enabling the rules here would ship a linter whose output is three-quarters another issue's backlog — the reliable way to teach everyone to ignore lint output. This enables recommended plus the two type-aware rules that catch defects rather than describe type debt, which leaves 110 findings.

Both configs downgrade every preset to a warning and then list the error rules explicitly at the bottom, so the CI gate is readable in one place instead of inferred from four presets' defaults. Errors are no-floating-promises, no-misused-promises, rules-of-hooks, exhaustive-deps and jsx-a11y/alt-text; everything else warns. No --max-warnings flag is needed because ESLint already exits non-zero on errors and zero on warnings. no-misused-promises runs with checksVoidReturn.attributes false, since onClick={async () => ...} is idiomatic React and safe when the handler catches its own errors — at the default it flags every antd button in the admin screens, 25 of its 28 hits, and a rule that is 89% noise gets switched off within a week.

The 37 errors this surfaced were mostly not the mechanical fix they looked like. The plan assumed the 30 floating promises were fire-and-forget loaders that already handled their own failures, which was true of the one sampled when writing the design and false for most of the rest: Admin, Categories, Customers, Tags, Settings, Account and CustomerAuthContext all had no rejection handling at all, so `void` on them would have hidden real failures rather than annotated deliberate ones. Each of those loaders now catches and surfaces the failure before the call site voids it. The CustomerAuthContext one was a live bug — a rejected fetchMe left loading true forever, rendering as a permanent spinner instead of a signed-out page.

Admin's load became a useCallback so its effect can name it honestly rather than suppress the dependency, Categories' drop handler was split so the function antd receives returns void as its type says, and Cart's effect now names refreshCartContext, which is a useCallback with an empty dependency list and so cannot re-run it. The only disable added is in asyncRoute, where returning a promise where Express expects void is the entire point of the wrapper and the promise cannot reject.

Two of the issue's premises did not survive measurement, both recorded in the spec: exhaustive-deps flags 2 cases rather than the 10 inferred from empty dependency arrays, and the backend was already clean on the defect rules because #59 wrapped every async route.

Verified: lint, build, 78 unit, 134 integration and 83 e2e all pass in both workspaces, and the CI gate was confirmed to fail by introducing a deliberate violation rather than by assuming the job is wired correctly.

Closes #60

frontend/package.json

@@ -5,6 +5,7 @@
   "scripts": {
     "dev": "vite",
     "build": "tsc && vite build",
+    "lint": "eslint src",
     "test:e2e": "playwright test"
   },
   "dependencies": {
@@ -18,13 +19,20 @@
     "remark-gfm": "^4.0.0"
   },
   "devDependencies": {
+    "@eslint/js": "^9.39.5",
     "@playwright/test": "^1.47.0",
     "@types/pg": "^8.23.1",
     "@types/react": "^18.3.3",
     "@types/react-dom": "^18.3.0",
     "@vitejs/plugin-react": "^4.3.1",
+    "eslint": "^9.39.5",
+    "eslint-plugin-jsx-a11y": "^6.10.2",
+    "eslint-plugin-react-hooks": "^7.1.1",
+    "eslint-plugin-sonarjs": "^4.2.0",
+    "globals": "^17.11.0",
     "pg": "^8.23.0",
     "typescript": "^5.5.4",
+    "typescript-eslint": "^8.67.0",
     "vite": "^5.4.0"
   }
 }