Downloadable releases, CI, and README refresh for discoverability

[kodinkod]

Why

The biggest adoption blocker right now isn't the README — it's that there is nothing to download. The only release is a single macOS-arm64 .dmg on a prerelease tag, so everyone else has to install Rust + Node and build from source. This PR makes "try Mycel in one minute" real and freshens the README around it.

What

Release automation (.github/workflows/release.yml)

• Tag-triggered (v*) workflow using tauri-apps/tauri-action: builds .dmg for macOS (Apple Silicon and Intel), .msi for Windows, .deb/.AppImage for Linux, and attaches them all to a draft GitHub Release for review before publishing.
• To cut a release: git tag v0.2.0 && git push origin v0.2.0, then publish the draft.

CI (.github/workflows/ci.yml)

• Frontend typecheck + build on every push/PR. (npm run lint is intentionally skipped — there is no ESLint config in the repo yet, so the script currently errors out; worth fixing separately.)

README

• Quick start now leads with "download a binary from the latest release", building from source becomes the second path; top badge links to releases.
• Documents two shipped-but-undocumented features: Present mode (⌘/Ctrl+Shift+P, --- slide splitting, overview, edit-from-slide) and LaTeX math (KaTeX, inline + block). Added to features, shortcuts table, and stack table.
• Removes per-note encryption and LaTeX from the roadmap — both have shipped (encryption already had its own README section, so it was listed twice with conflicting status).

Notes

Verified locally: npm run build (tsc + vite) passes. The release workflow can only be exercised by pushing a tag.

https://claude.ai/code/session_017WeqJfUrTJV3HGiB4b8JZF

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