Apply smooth corners to inline media

[klopez4212]

Summary

• Add a local smooth-corner utility with the 0.6 smoothing behavior from the Tailwind reference, without adding a dependency.
• Apply the same smoothed radius to inline images, galleries, videos, code blocks, and rich attachment cards.
• Keep the existing radius scale while making media/modal radius transitions visually consistent.

Snapshot

Checks

• git diff --check HEAD
• pnpm --dir desktop check
• pnpm --dir desktop typecheck
• Combined validation before splitting: pnpm --dir desktop test, pnpm --dir desktop build, focused Playwright smoke/integration suites