v0.8.48 — more retries for preset buttons on custom streams

Preset buttons on custom streams now get up to three tries instead of one on the SoundTouch 10.

On SoundTouch 10 speakers with firmware 27.0.6, pressing the button of a
custom-stream preset while the speaker is in standby does not start
playback on its own, and the speaker then usually hangs: the preset LED
keeps blinking and the power button is ignored. Since v0.8.23 SixBack
re-sends the station 5 seconds after such a press, which starts playback —
but it did so only once. If that single attempt did not take, the speaker
stayed hung until it was unplugged.

What changed:

• After re-sending the station, SixBack now checks whether playback
  actually started. If not, it sends the station up to two more times,
  10 seconds apart (longer while the speaker is still buffering).
• It stops as soon as the speaker plays, is switched off, changes to
  another source or preset, or the web interface starts something itself.
  Pressing the same button again does not interrupt it.
• Switching the speaker off within the first seconds after a press no
  longer lets the retry switch it back on.

Verified on a SoundTouch 10 with firmware 27.0.6, including presses of the
physical button.

Applies to the ESP32-S3 builds (the hardware-button watcher needs PSRAM).
No partition changes — this release can be installed over the air.