* ci: reduce PR check time from ~30 min to ~1 min
- merge lint + prettier into single code-quality workflow
- add concurrency groups to cancel stale runs on rapid pushes
- add path filters so irrelevant changes skip checks
- make full nix build opt-in via "nix" label (eval-only by default)
- add nix store caching via magic-nix-cache-action
- cache trusted-signing-cli binary on windows builds
- upgrade setup-bun from v1 to v2
* ci: add workflow path filters and expand nix build triggers
- add .github/workflows/** to path filters on code-quality and nix-check
so CI runs when workflow files themselves change
- include tauri.conf.json and build.rs in nix full-build diff check since
these can break nix sandbox builds independently of cargo builds
