test(expo-template): make tests and checks runnable

[oleksandrzavarzin-callstack]

PR Description

The Expo template's only test failed on a fresh scaffold, the script running it couldn't report
that failure, and nothing linted or typechecked the template at all.

• The test was broken. ThemedText-test.tsx used react-test-renderer, which returns null
  from .toJSON() under React 19, asserted against a snapshot generated by an older React. Rewritten
  with @testing-library/react-native v14 and the stale snapshot is deleted.

• "test": "jest --watchAll" never exited. With CI=true and stdin closed it ran the suite,
  printed the interactive watch hint, and parked forever waiting for a keypress; no exit code, so a
  failing test reported nothing. Split into test / test:watch.

• No lint, no typecheck. The template defined neither script and shipped no eslint config.
  Added lint (expo lint) and typecheck (tsc --noEmit), plus the eslint.config.js.

• 4 lint errors in the template's own copy (react/no-unescaped-entities on apostrophes) are
  escaped, so npm run lint is green on a fresh scaffold.

yarn lint / yarn typecheck at the repo root now skip this package. That's deliberate: the root
pins @types/react through resolutions while the template declares its own, so checking it
in-place reports results no user ever sees. CONTRIBUTING.md explains this and documents the
pack-and-scaffold flow instead.

Important

Merge after #577 - align dependencies with Expo SDK 54>. This branch is off main, so the two
type errors that PR fixes are still present here; until it lands, the typecheck script this PR
adds fails on a scaffolded app.

How to test?

yarn workspace @bottom-tabs/expo-template pack --out /tmp/expo-template.tgz
npx create-expo-app@latest /tmp/tpl --template /tmp/expo-template.tgz
cd /tmp/tpl && npm install

• npm run lint → exit 0
• CI=true npm test → exits, and exits 0 (before: hung indefinitely)
• npm run typecheck → clean, once PR fix(expo-template): align dependencies with Expo SDK 54 #577 is in.
• yarn lint && yarn typecheck at the repo root still pass, with the template excluded.

Screenshots

Not applicable - no runtime or UI change.