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>