feat(web): notes picker reaches C6, plus a chord mode

[benjamin-small]

Summary

• Picker spans C3–C6 (37 chips, one row per octave).
• New Pick select: Single notes (default) or a triad — Major, Minor, Diminished, Augmented, Sus2, Sus4 (web/src/lib/notes/chords.ts).
• In chord mode a chip press replaces the pick with that note's root-position triad; pressing the root again clears it; roots whose triad would pass C6 are aria-disabled with a hint. Changing the chord re-voices the current root; switching back to Single notes keeps the pick.
• Share links carry &c=<chord>; unknown ids are dropped; Reset returns to Single notes.
• No Rust changes — the engine already takes any three notes.

Validation

• cargo fmt --check, cargo clippy, cargo test pass (clippy's one warning is pre-existing, in untouched code)
• vitest 391/391 (new chords.test.ts + 9 chord-mode component tests), svelte-check 0 errors
• Browser: #/notes?n=57&c=minor → pressing D4 gives D4/F4/A4 and ?n=62,65,69&c=minor; F♯5 and above greyed out; no console errors