fix(web): surface 4xx + UI banner when evaluation start fails (#280)

[DerDennisOP]

Summary

• Stops core::sources::git::check_for_updates from swallowing ls_remote_head failures; the polling loop already handles Err(e) gracefully.
• Maps the propagated SourceError to 400 with new code: "repository_unreachable" on POST /projects/{org}/{p}/evaluate and POST /projects/{org}/{p}/check-repository, dropping the old commit_hash.is_empty() → 500 fallback. Message includes the underlying git error.
• Frontend ProjectDetailComponent now stores the API error in an errorMessage signal and renders an inline, dismissible .evaluation-error banner under the project header instead of failing silently.

Closes #280.

Test plan

• cargo check --workspace --tests clean
• cargo clippy --workspace --tests -- -D warnings clean
• npx tsc --noEmit clean
• CI: cargo test -p core --test git_remote (new test check_project_updates_propagates_unreachable_remote_error)
• CI: pnpm --dir frontend exec ng test --watch=false (new specs in project-detail.component.spec.ts)
• Manual reproduction from the issue (invalid github:nix-community/bun2nix URL): backend returns 400 repository_unreachable; UI banner shows the underlying git error.

[DerDennisOP]

/gradient run

[DerDennisOP]

/gradient run checks.x86_64-linux.gradient-cache