Resolve PR review feedback and unblock GitHub Pages preview deploy

[Copilot]

This PR addresses outstanding review feedback on the Astro migration branch and removes the deploy-time blocker in the PR preview workflow. Scope is limited to base-path correctness, static output collisions, and workflow permission alignment.

• Base-path correctness in runtime data and metadata

  • Remote snapshot fetch path now derives from import.meta.env.BASE_URL instead of a root-absolute path, so sub-path deployments (e.g. Pages) resolve correctly.
  • Reintroduced favicon metadata in BaseLayout using a base-aware href.
  • Updated navigation comments to match current behaviour (window.location.assign), removing stale pushState references.

• Static output collision removal

  • Moved fallback page from public/index.html to public/fallback/index.html to avoid overwriting Astro-generated /index.html.
  • Updated repository references that pointed to the old fallback location.

• Dependency surface cleanup

  • Removed unused direct dependency zod from package.json (and lockfile).

• Preview deploy workflow unblock

  • Added pull-requests: write to .github/workflows/github-pages-preview.yml permissions to support PR preview deployment operations.

// src/hooks/useRemoteData.ts
const REMOTE_DATA_PATH = `${import.meta.env.BASE_URL}__remote-data`;
const response = await fetch(`${REMOTE_DATA_PATH}/${resource}.json`, { /* ... */ });

# .github/workflows/github-pages-preview.yml
permissions:
  contents: read
  pages: write
  id-token: write
  pull-requests: write