fix(ci): generate desktop marketplace JSON in nightly build

[siracusa5]

Problem

The nightly build has failed every night since 2026-07-28 (4 consecutive runs). nightly.yml's build-desktop job never generated apps/desktop/src/lib/marketplace/marketplace.generated.json — a gitignored build output — so tsc died:

src/lib/marketplace/data.ts(8,23): error TS2307: Cannot find module
'./marketplace.generated.json' or its corresponding type declarations.

This is the same class of bug #323/#324 fixed in release.yml and validate.yml. Those two workflows got the fix; nightly.yml was a third hand-rolled copy of the same recipe and was missed. Nightly is the only one of the three not exercised on PRs, so the drift went unnoticed.

Changes

1. New composite action .github/actions/prepare-desktop-frontend — builds shared/core/design-tokens/ui and generates the desktop marketplace JSON. Takes a strict input, since validate.yml intentionally runs the generator non-strict.
2. All three desktop-building jobs call it instead of duplicating the steps (nightly.yml, release.yml, validate.yml). One place to change means the copies can't drift apart again.
3. New report-failure job in nightly.yml — on a scheduled failure it opens (or comments on) a nightly-failure issue. This is the part that stops a silent recurrence: the breakage was visible in Actions for four days and nobody was told.

Verification

Deleted marketplace.generated.json locally, ran the composite action's exact steps, then pnpm --filter harness-kit-desktop build — the tsc && vite build that failed in CI. Passes, built in 13.9s. All four YAML files parse.

The real proof is a workflow_dispatch run of Nightly off this branch once CI is green.

🤖 Generated with Claude Code