docs: document build trigger in frontend spec

[Scr4tch587]

What

Documents the build trigger in SPEC-frontend.md: new "Build panel" feature section, build-panel.tsx in the file structure, and triggerBuild/dryRunBuild in the api-integration list (with the note that browsing still never builds — builds only happen explicitly through the panel).

Why

Repo convention: spec files track what the code actually does, updated as the final PR of the stack.

Benefit

The frontend spec stays an accurate map of the shipped UI for the next person touching it.

[github-actions]

• main
  • feat: replace svelte frontend with react + tailwind scaffold #375
    • feat: typed api client and api key handling #376
      • feat: dataset browsing ui #377
        • ci: deploy frontend to github pages #378
          • docs: update specs for react migration #379
            • feat: github rest client for dataset proposals #383
              • feat: dataset proposal service and endpoint #384
                • feat: api client support for dataset proposals #385
                  • feat: dataset creation ui #386
                  • docs: document dataset proposal flow in specs #387
                  • feat: api client support for triggering builds #388
                  • feat: build trigger ui #389
                  • docs: document build trigger in frontend spec 👈