fix: refine header quick actions and throughput

[numnx]

Summary

• Moves Web App and Desktop App setup from top-nav buttons into idle 3D chat quick actions that operate on the current project and current techstack selection.
• Keeps unassigned existing/imported project techstacks as None, while preserving the explicit new-project default behavior.
• Reworks the header token-throughput signal to a compact app-wide 20-second live activity view with a 20-point rolling line, stable width, and corrected active-duration rate calculation.
• Renames the built-in stack label to Code UX Stack and updates affected docs/tests.
• Preserves existing project override sections when the top-nav techstack selector saves, so Jira, agents, QA, MCP, memory, worker, and provider settings are not overwritten by a techstack-only change.

Root Cause

The app setup actions were integrated into the header instead of the idle chat quick-action surface. The initial 20-second throughput implementation also normalized cumulative invocation totals over the wall-clock window, which inflated long-running invocations and made the header graph appear permanently increasing.

A follow-up bug was found in the top-nav techstack save path: the frontend sent a partial { techstack } object to a replace-style project settings endpoint. The fix now loads the current project override and merges only the techstack fields before saving.

Validation

pnpm vitest run tests/dashboard/v2/top-nav-token-throughput.test.tsx tests/backend/repositories/execution/header-token-throughput-query.test.ts tests/dashboard/lib/header-token-throughput.test.ts
pnpm vitest run tests/dashboard/lib/settings.test.ts tests/backend/repositories/settings-repository.test.ts tests/dashboard/v2/top-nav-techstack.test.tsx tests/dashboard/v2/top-nav-browser-menu.test.tsx dashboard/src/v2/components/settings/__tests__/SettingsControls.test.tsx
pnpm vitest run tests/dashboard/v2/settings-api.test.ts tests/dashboard/v2/top-nav-techstack.test.tsx
pnpm run lint
pnpm run build

Additional local checks:

• Dashboard health endpoint returned UP on http://localhost:4444/health.
• Playwright DOM check confirmed the header throughput line keeps 20 points, stays green, and does not restart stroke-dash redraws each second.

Risk / Rollback

Risk is limited to dashboard header/chat quick-action behavior, compact header throughput, and project techstack selector saving. Roll back this PR to restore the previous top-nav action placement and throughput visualization.