Implement searchable FAQ feature with article validation and tests

[directorfloo]

Closes #566

Here's a copyable PR description you can paste into GitHub:

Title: Add Help Centre (searchable FAQs, ratings, no-results tracking)

Summary:
Adds a lightweight, client-side Help Centre with full-text search (Fuse.js with substring fallback), categorized articles, per-article "Was this helpful?" ratings (stored in localStorage), and tracking of popular no-result queries. Includes unit tests and a lightweight validation script.

Files changed / added:
- Updated: `frontend/package.json` — add `fuse.js` dependency
- Added: `frontend/src/content/help/articles.json` — sample FAQ articles
- Added: `frontend/src/lib/help-search.ts` — search utility (Fuse.js + fallback)
- Added: `frontend/src/app/help/page.tsx` — Next.js client page and UI
- Added: `frontend/src/__tests__/help-search.test.ts` — unit tests for search
- Added: `frontend/scripts/validate-help.js` — quick validation script

Why:
- Reduce support load by surfacing common answers and letting users self-serve.
- Capture no-result queries to identify documentation gaps.
- Provide a small, incremental MVP that can be upgraded to Algolia DocSearch or server-backed tracking later.

Acceptance criteria coverage:
- Full-text search across articles: implemented via Fuse.js, with substring fallback if Fuse not installed.
- Articles organized by category: grouped and displayed when no search query.
- Each article has a `Was this helpful?` rating stored in localStorage under `helpful_<articleId>`.
- Popular search queries with no results are tracked in localStorage under `help_no_results`.

How to run & test locally:
1) Quick validation (no install required):
   cd frontend
   node scripts/validate-help.js

2) Full test run (requires installing deps):
   cd frontend
   npm install
   npm test

How to review:
- Check article text and categories in `frontend/src/content/help/articles.json`.
- Open the Help UI at `/help`:
  - Confirm search returns relevant articles.
  - Confirm browsing by category lists grouped articles.
  - Click `Yes`/`No` and verify `localStorage` entries `helpful_<articleId>`.
  - Search gibberish and verify `help_no_results` in `localStorage`.
- Run tests or the quick validator above.

Notes & next steps:
- Ratings and no-results are stored locally for MVP. For aggregated analytics we can POST to a server endpoint or integrate with existing telemetry.
- UI uses inline styles for rapid iteration; can be refactored to use the design system components.
- Articles are currently JSON; we can convert them to MDX for richer formatting if desired.

Rollback:
- Remove the added files and dependency or revert this branch/PR.

Would you like me to:
- run `npm install` and the full test suite now and fix any failures, or
- convert the articles to MDX and add MDX rendering?

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