Add downloadable file export for generated templates

[zanni098]

Summary

Add a one-click export that downloads the generated issue form, config, and triage checklist as files.

Why

Right now the app is useful, but copying three outputs manually adds friction for first-time maintainers.

Done when

• users can download the generated outputs as files
• filenames are sensible for the chosen project name
• the README mentions the export flow

[zanni098]

Planning to implement this in the next update. The approach will be:

1. Generate a .zip bundle containing the issue form YAML, config, and triage checklist
2. Use client-side Blob + URL.createObjectURL for zero-backend downloads
3. Filenames will follow the pattern: {project-slug}-bug-report.yml, {project-slug}-config.yml, {project-slug}-triage.md

This should land alongside the multi-template work in #1.