Modernize desktop UI and remove misleading image-editor affordances

[dovvnloading]

Summary

• Rework the desktop shell as a compact local workbench: matte canvas, focused thread rail, text-led settings, and a centered composer island.
• Remove the dedicated image-transformation panel, starter row, composer action, related CSS, route wiring, and copy from the chat UI.
• Preserve image operations in the execution layer for approved code-driven tasks without presenting Cortex as an image editor or generator.
• Fix new-thread navigation so the prior conversation is cleared from the active selection.
• Harden packaged startup for windowed Uvicorn logging and prevent packaging when a frontend build fails, which could otherwise ship a stale bundle.
• Add the full UI audit and release-gate documentation.

Why

Cortex is a local chat and agentic workbench. The prior chat surface advertised a first-class image editor that the product does not provide. This change keeps the underlying execution capability available for deliberate code-driven work while making the product boundary clear in the UI.

Validation

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

Scope note

The existing user-owned Cortex.pyproj worktree change was intentionally excluded from this commit and PR.