ci(dashboard): verify routeTree.gen.ts is regenerated

[pratyush618]

Summary

Adds a guard step to the dashboard CI workflow that fails the build if dashboard/src/routeTree.gen.ts differs from the freshly-generated tree.

The composite dashboard-build action already runs pnpm exec tsr generate before typecheck/test/build, so a stale committed routeTree.gen.ts would silently be overwritten in CI. Without this check a developer can add a route, forget to regenerate, and have the build still pass — leaving the committed tree drifted from source.

The step runs only in the dashboard PR/push workflow (dashboard.yml), not in the publish workflow, so release builds aren't blocked by drift detection.

Addresses recommendation 11 from the 2026-05-08 audit (.claude/reports/audit-2026-05-08.md).

Test plan

• Local: re-running pnpm exec tsr generate against the current source produces no diff against the committed file.
• CI: this PR exercises the new step; expect green.
• Manual: add a stub route, push without regenerating, observe the step fail with the helpful error message.