feat: add automated release pipeline with release-please

[rubenhensen]

Summary

• Replace standalone Docker build workflow with combined CI pipeline using release-please for automated version bumping, changelog generation, and GitHub releases
• Build Docker images on native multi-platform runners (amd64 + arm64) instead of QEMU emulation, using the digest-based manifest pattern from the cryptify repository
• Move SvelteKit frontend build into the Dockerfile (new Node.js stage) so the Docker build is self-contained and works with per-platform native runners
• Add x-release-please-version marker to .env so VITE_APP_VERSION stays in sync with releases
• Align package.json version to 1.0.0 to match the deployed .env version

How it works

On push to main, the pipeline:

1. release-please checks conventional commits and either creates/updates a release PR, or (when a release PR is merged) creates a GitHub release + tag
2. build jobs run on native amd64/arm64 runners, pushing images by digest
3. finalize merges digests into a multi-platform manifest with tags:
   • edge on every push to main
   • pr-123 on pull requests
   • 1.2.3 when release-please creates a release

This mirrors the CI pattern used in cryptify. release-please was chosen over release-plz (Rust-specific) as the standard equivalent for Node.js projects, following the same conventional-commits + release-PR workflow.

Closes #57

Test plan

• Verify the CI workflow runs successfully on this PR (build jobs for amd64/arm64)
• Merge to main and confirm edge tag is pushed to GHCR
• Verify release-please creates a release PR after merge
• Merge the release PR and confirm a versioned Docker image tag appears