feat(web): picker stops at B5; chord mode is a row of radio buttons

[benjamin-small]

Summary

• No C6: the picker is C3–B5, 36 chips in three full octave rows (row break before every C except the first). Chord roots that would pass B5 are greyed out with "… runs past B5"; links drop midi above 83.
• Radio chord picker: the Pick dropdown is now a radiogroup of real type="radio" inputs (shared name, so arrow keys move between them), drawn as pills like Pure ratios / Piano with the chosen one tinted and a focus ring on keyboard focus. Behaviour unchanged: a chord re-voices the current root; Single notes keeps the pick.

Validation

• vitest 396/396 (chord-mode tests now click radios and assert a single shared group name; range and limit tests moved to B5), svelte-check 0 errors, cargo fmt clean (no Rust changes)
• Browser: 36 chips ending at B5; clicking Minor then D4 gives D/F/A at 0.50 → 0.56 Hz; ArrowRight moves to Diminished and re-voices to D/F/A♭; F♯5 and up greyed for Diminished; no console errors