fix(mobile): guard against OOM crash on Load, fix stuck Preparing audio - #235

Instead of blocking long tracks with an error, tracks that would exceed
200 MB of decoded PCM (approx 4.5 min x 4 stems) now fall back to a
streaming engine backed by <audio> elements and createMediaElementSource.
No PCM is held in RAM -- the browser streams on demand -- so OOM crashes
are avoided without restricting track length.

The streaming engine implements the same interface as createAudioEngine
(play/pause/seek/setGain/destroy/ready) so openTrack needs no structural
changes beyond selecting which engine to create.

Play button: add data-playing attribute and CSS so the button turns green
when playing (gold when paused), making the state immediately obvious.

Seek desync: the streaming engine was setting currentTime on all <audio>
elements while they continued playing, causing each stem to arrive at the
new position at a slightly different time. Fix: pause all elements first,
seek all, then resume -- this guarantees all stems restart from the same
position simultaneously.

… style

Prev/next buttons were rendered but had no data-action and no handler --
clicking them did nothing. Now:

- prevTrack() / nextTrack() navigate state.tracks by index, autoplaying if
  a track was already playing when the button was pressed
- Buttons are disabled at the ends of the library (no prev on first track,
  no next on last track)
- .t-step CSS updated to match the desktop daw-iconbtn style: transparent
  background, rounded corners, hover highlight, scale(0.96) on active

Two-part fix for stems going out of sync after dragging the playhead:

1. Seek: pause all elements, set all currentTime, then wait one RAF frame
   for the browser to settle the seeks. Re-read the primary element's
   actual position and align all secondaries to it before calling play().
   This eliminates startup desync caused by elements buffering at different
   rates after a seek.

2. Drift correction: tick() now checks secondary elements every ~1 second
   (60 RAF frames) and snaps any that have drifted more than 50ms back to
   the primary's position. Catches any clock skew that accumulates during
   long playback.

…er style

Remove the rectangular box shape (border-radius:9px) that clashed with the
circular play button. Prev/next are now naked icon buttons with a circular
tap area -- the pattern used by Spotify, Apple Music, etc. Icons enlarged
from 26px to 32px to sit proportionally next to the 66px play button.
Press feedback is a scale+opacity pop instead of a subtle background fill.

The streaming engine (<audio> elements via createMediaElementSource) caused
constant choppy audio -- the exact same HTTP/1.1 connection-cap underrun
issue that audioEngine.js was built to solve in the first place.

Replace with the buffer engine for all tracks. Raise the decoded-PCM limit
from 200 MB to 600 MB, which covers ~14 min x 4 stems at 44.1 kHz/Float32
-- well within the per-tab budget of any post-2019 phone. Tracks over that
threshold get a clear error rather than glitchy playback.

…esting

…ktop