[codex] add empty state for project filters

[Sean-Kenneth-Doherty]

Summary

• add a hidden empty-state message below the project list
• reveal it when search/tag filtering hides every project card
• keep the initial all-projects view unchanged

Addresses #501

Validation

• git diff --check
• extracted inline script from src/components/ProjectList.astro and ran node --check -
• GITHUB_TOKEN=$(gh auth token) pnpm build
• generated dist/index.html contains the hidden #no-results element
• source check confirms the filter reads #no-results and toggles hidden based on the filtered project count

[Sean-Kenneth-Doherty]

Validation notes for this UI/UX slice:

• git diff --check passes
• extracted the inline script from src/components/ProjectList.astro and node --check - passes
• GITHUB_TOKEN=$(gh auth token) pnpm build passes
• generated dist/index.html includes the hidden #no-results element with No matching projects found.
• source check confirms the filter reads #no-results and toggles its hidden state when filteredProjects.length changes