refactor: from astro to vite+preact & radically simple

[gildesmarais]

This pull request updates project documentation and developer tooling to reflect the migration of the frontend from Astro to Preact (built with Vite), and introduces improvements to the API feed creation boundary and CI workflow. The changes modernize the stack, clarify documentation, and strengthen type safety and API contract boundaries.

Frontend migration and documentation updates:

• Updated all references in .github/copilot-instructions.md and README.md from Astro to Preact and Vite, including architecture descriptions, dev server instructions, and documentation links. [1] [2] [3] [4] [5] [6] [7]
• Updated Makefile targets and descriptions to refer to the Preact frontend, replacing Astro terminology. [1] [2] [3]
• Updated Dockerfile to copy the new frontend build output location.

Continuous Integration and developer tooling:

• Added a frontend typecheck step (npm run typecheck) to the CI workflow and Makefile, ensuring type safety for the Preact frontend. [1] [2]
• Added check-frontend and quick-check Makefile targets for streamlined local and CI checks, and improved the ready target to include these checks. [1] [2]
• Renamed CI and Makefile build steps for clarity (e.g., "Build frontend static assets"). [1] [2]

API feed creation boundary improvements:

• Refactored app/api/v1/feeds/create_feed.rb to clarify and simplify the API boundary: removed redundant comments, introduced FEED_ATTRIBUTE_KEYS for explicit response shaping, and factored out feed metadata normalization into dedicated private methods. [1] [2] [3] [4] [5] [6] [7] [8]
• Moved the extract_site_title logic to the API edge (feeds.rb) and updated it to use Html2rss::Url directly for improved encapsulation. [1] [2] [3]

Other improvements:

• Cleaned up Makefile and Dockerfile to remove obsolete Astro-specific artifacts and references. [1] [2]

These changes collectively modernize the stack, improve developer experience, and clarify the API and documentation.