refactor: fix browser tab title to brand and drop SITE_NAME

Related Issue

N/A

Changes

• Fix the browser tab title to the brand name showmycode on every page (previously it followed SITE_NAME → GITHUB_OWNER)
• Update the site description metadata to reflect showmycode's purpose as a general-purpose private-repo sharing tool, not an interviewer-only viewer
• Remove the SITE_NAME env var — it was used in a single place (home header) and only duplicated the GitHub owner shown on repo pages. The home header brand now derives directly from GITHUB_OWNER, keeping all headers consistent
• Fail fast in getAllowedRepos() with "GITHUB_OWNER is not set" instead of silently producing a confusing GitHub 404 downstream
• Sync env docs: drop SITE_NAME from .env.example and CLAUDE.md, and add the previously-undocumented SHARE_TOKEN to the CLAUDE.md env list

How to Verify

1. Run npm run dev with a valid .env.local
2. Open any page — the browser tab title reads showmycode
3. Home and repository headers both show the GitHub owner consistently
4. Temporarily unset GITHUB_OWNER and reload — the app fails with a clear GITHUB_OWNER is not set. See .env.example. error (not a 404)

Checklist

• npm run format:check passes
• npm run lint passes
• npm run typecheck passes
• npm run build passes
• PR title follows Conventional Commits (feat:, fix:, chore:, refactor:, docs:, test:, )