Removes unused JavaScript files, exports, and npm dependencies identified during a dead code sweep.
- Deleted `js/healthService.js` (unused).
- Deleted `js/nostr/signer-interface.js` (unused).
- Deleted `js/watchHistorySort.js` (unused).
- Deleted `js/services/signerKeyManager.js` and `tests/ui/signerKeyManager.test.mjs` (unused).
- Removed `buildStorageChallengeEvent` from `js/nostrEventSchemas.js`.
- Removed `path` and `webtorrent` from `package.json` dependencies (unused/vendored).
- Updated `tests/hashtag-preferences.test.mjs` to fix a mock-related failure exposed by environment checks.
Verified with `npm run test:unit`. `lint` and `format` checks run.
Note: `tests/moderation-service.test.mjs` failure deemed pre-existing.
See `artifacts/depcheck.json` (if retained) for dependency analysis.