Remove preset chat starter suggestions

[dovvnloading]

Summary

• Remove the preset prompt cards from the blank-thread view.
• Leave the new-thread surface intentionally quiet: title, concise context, model status, and the real composer are the only entry points.
• Remove the associated starter-list styles and replace the click-through test with an explicit no-presets regression test.
• Document the product decision in the UI modernization audit.

Why

Cortex is a local workbench, not a demo template gallery. Preset suggestions made the first screen feel generic and implied a guided workflow that the user did not ask for. A blank thread should give the user control of the first message and keep attention on the composer.

Validation

• 44 frontend component tests (single-worker run)
• 6 Playwright browser flows
• Frontend typecheck, lint, and production build
• Fresh Windows package build succeeded
• Packaged launch returned GET /api/v1/health/ready with HTTP 200
• Production bundle contains no starter-card selectors or labels
• git diff --check clean

Scope note

The existing user-owned Cortex.pyproj worktree change was intentionally excluded from this commit and PR. This PR targets the already-merged UI modernization baseline from PR #66.