fix mobile layout issues #140

Implemented the mobile layout refinement for Radial’s space picker.

• Turns the narrow space-switcher overlay into a top-aligned, edge-to-edge sheet so long space lists remain reachable instead of being vertically centered out of view.
• Adds safe-area-aware padding for standalone and overlay picker states, and tightens mobile identity, row, and private-badge spacing.
• Applies a consistent 16px editing-text floor at phone widths, including browsers or test environments that do not reliably report coarse pointer capabilities.
• Adds regression coverage for the mobile sheet geometry, safe-area handling, and field-size tokens.

Checks:

• pnpm build
• pnpm --filter @radial/ui typecheck — 0 errors and 0 warnings
• pnpm --filter @radial/ui test — 95 Node tests and 860 Vitest tests passed

Remaining risks: none known. The UI build correctly reported that the optional browser iroh WASM binding was not present in this checkout; this is unrelated to the CSS-only change.

Radial artifact