v0.8.47 — a way back when an update is interrupted

A way back when an online update is interrupted.

The update downloads the web interface first and writes it in place, so a
connection that drops midway leaves that part half-written and the page
gone. Everything else keeps running — the firmware, the speakers, every
stored preset. Until now the device could only answer with a bare list of
API endpoints, which reads as "broken" and pointed people at the web
flasher, where erasing the device sits next to updating it.

What changed:

• The page shown when the web interface is missing now states what
  happened, confirms that speakers and presets are unaffected, and offers
  a "Reinstall web interface" button that downloads the missing files
  again and shows the progress. It also warns that choosing Install /
  erase in the web flasher is a factory reset and loses the Wi-Fi
  credentials and all presets. Builds that cannot update themselves point
  to the manual upload and to "Update existing" instead.
• After a failed update the install button is no longer greyed out, so an
  update can simply be tried again, and the error says that the previous
  firmware is still active.
• "Send to maintainer" in the snapshot dialog is enabled again when the
  dialog is reopened.

Verified on hardware by interrupting a real download and repairing the
device from the button in a browser.

No partition changes — this release can be installed over the air.