Playback speed: drop 0.25x and move 0.5x to 0.75x #433

The transport offers 0.25x / 0.5x / 1x (SPEED_PRESETS in static/js/transport.js).

0.25x is too slow to be musically useful: at that rate the time-stretch artefacts dominate and the part is harder to follow, not easier. 0.5x is a big jump from 1x for most practice work.

Change the presets to 0.75x / 1x:

• remove the 0.25x button
• move 0.5x to 0.75x

Touches SPEED_PRESETS, the speedBtns id list in static/js/state.js, and the three buttons in static/index.html. No i18n keys change (the labels are numeric).

Note the mobile UI uses a continuous 0-2 slider with a 0.25 step rather than presets, so it is unaffected.