Align shared form control heights

[OwenMcGirr]

Summary

• add a shared 3rem height token for single-line controls
• normalize inputs, selects, and buttons so their boxes align exactly
• keep buttons flexible so wrapped labels can grow, while leaving textareas independently sized

Why

Native selects, text inputs, and buttons used different intrinsic line-height calculations. In the repository toolbar this produced 48.3–48.8px fields beside 44px buttons, leaving the button tops visibly lower despite shared bottom alignment.

Test plan

• pnpm lint (passes with 3 existing Fast Refresh warnings)
• pnpm typecheck
• pnpm --filter @opensymbols/site test (65 tests)
• pnpm test (site/server, PostgreSQL integration, and 26 legacy tests)
• pnpm build
• docker compose config --quiet
• git diff --check
• browser geometry: all five repository controls render at 48px with identical top and bottom coordinates
• responsive browser checks at 800px, 480px, and 320px with no horizontal overflow

True browser-level 400% zoom and forced-colors rendering remain manual reviewer checks; the equivalent narrow-width reflow was verified at 320px.

Scope and risk

This changes only shared control sizing. Component interfaces, form behavior, responsive breakpoints, textareas, and PR #54 are unchanged.

Closes #67