test: component and hook unit tests for the web app

Ported from the amal66 fork (index: Open-Legal-Products/mike#205), adapted
to this repo's frontend/ layout and current component behavior. Adds the
jsdom + testing-library harness on top of the existing vitest setup:
vitest.config.mts (jsdom environment, @/ alias mirroring tsconfig paths,
dummy Supabase env for modules that build a client at import time) and
vitest.setup.ts (jest-dom matchers). New devDependencies: jsdom,
@testing-library/react, @testing-library/jest-dom,
@testing-library/user-event, @vitejs/plugin-react.

Ported suites (30 new tests):
- FileTypeIcon.test.tsx (11) — fileTypeKind mapping + icon rendering
- TRTable.test.tsx (1) — header/row render smoke test; the fork's ARIA
  role assertions (table/columnheader, "Tabular review" label) target
  fork-only markup — this repo's grid is div-based, so the test asserts
  rendered content instead
- button.test.tsx (4), pill-button.test.tsx (8), cite-button.test.tsx (3)
- useSmoothedReveal.test.ts (3) — passes against this repo's early-return
  snap behavior unchanged

Dropped (subjects don't exist here):
- HistoryDropdown.test.tsx — no tr-chat-panel/HistoryDropdown component
- applyAssistantStreamEvent.test.ts — no such module
- useProjectsQuery/useTabularReviewsQuery/useWorkflowsQuery tests —
  react-query is fork-only
- lib/toast.test.ts — no frontend/src/lib/toast.ts
- useAssistantChat.parsers.test.ts — the parser helpers exist inside
  useAssistantChat.ts but are not exported, and the fork's extracted
  module doesn't exist here

Verified: frontend npm test 38/38 passing (30 new + 8 pre-existing cn()
utils tests); npx tsc --noEmit clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/package.json

@@ -62,15 +62,20 @@
     },
     "devDependencies": {
         "@tailwindcss/postcss": "^4",
+        "@testing-library/jest-dom": "^6.9.1",
+        "@testing-library/react": "^16.3.2",
+        "@testing-library/user-event": "^14.6.1",
         "@types/marked": "^5.0.2",
         "@types/node": "^20",
         "@types/react": "^19",
         "@types/react-dom": "^19",
         "@types/uuid": "^10.0.0",
+        "@vitejs/plugin-react": "^4.7.0",
         "babel-plugin-react-compiler": "1.0.0",
         "baseline-browser-mapping": "^2.9.11",
         "eslint": "^9",
         "eslint-config-next": "^16.2.6",
+        "jsdom": "^27.0.1",
         "tailwindcss": "^4",
         "tw-animate-css": "^1.4.0",
         "typescript": "^5",