feat: build trigger ui

[Scr4tch587]

What

Adds build-panel.tsx to the top of the dataset detail view: start/end date pickers (default last 30 days), a dry-run checkbox, and a build button wired to the api client from #388.

• Real build: success toast + invalidates the dataset's data query and the dataset list, so the table and the has_data dot refresh without a reload
• Dry run: produced rows render inline in a DataTable (capped at 50 timestamps) with the JSON modal per row, so builder output can be inspected before committing anything
• Pending state: button disables with a note that builds run synchronously server-side (the request stays open for the whole build)
• Errors: backend detail renders in an inline error box; 401s go through the central handler

Why

Builds could only be triggered via curl; browsing intentionally never builds. The default window is smaller than the 5-year browse window because builds are heavy.

Benefit

Datasets can be built and validated (dry-run first, then for real) entirely from the UI.

[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 👈
                  • docs: document build trigger in frontend spec #390