Add built-in audio support, modify/uninstall scripts and Trixie fix

[ermanno00]

Summary

This PR extends the installer in three directions while keeping the
existing UX intact:

1. Built-in audio is now a first-class option. The device picker no
   longer hides bcm2835, Headphones or vc4-hdmi — users can drive
   a speaker from the 3.5mm jack or HDMI without having to dismiss the
   "no DAC found" fallback prompt.
2. Two new management scripts so users don't have to reinstall from
   scratch to rename the speaker, swap the audio device or remove the
   stack.
3. A unified menu wrapper that dispatches to install / modify /
   uninstall based on what's currently on the system.

It also fixes a build failure on Raspberry Pi OS / Debian 13 "Trixie",
where plistutil was split out of libplist-dev into its own package.

Changes

install_airplay_v3.sh

• select_audio_device(): removed the grep -iv 'bcm2835\|Headphones\|vc4-hdmi'
  filter. All cards returned by aplay -l are listed and tagged
  [built-in] or [external/DAC] so the user can clearly tell them
  apart. Single-device auto-select is preserved.
• Header comment and ASCII banner updated to reflect support for USB
  DAC, audio HAT or built-in audio.
• Added libplist-utils to the APT dependency list. On Trixie the
  plistutil binary needed by shairport-sync's --with-airplay-2
  configure step lives in this separate package, and the build fails
  with configure: error: plistutil can not be found without it.

New: modify_airplay.sh

Interactive menu to modify an existing installation without rebuilding:

• Change AirPlay device name
• Change audio output device (re-uses the same labelled picker)
• Change ALSA mixer / hardware volume control (or disable it)
• Change volume limits (volume_max_db, default_airplay_volume)
• Test audio output via speaker-test
• View configuration, show service status, restart, edit with nano

Changes are written to /etc/shairport-sync.conf via targeted sed
rules that match both commented and uncommented forms, and the service
is restarted automatically after each change.

New: uninstall_airplay.sh

Clean removal of everything the installer adds:

• Stops and disables shairport-sync and nqptp
• Removes binaries from /usr/local/bin
• Removes /etc/shairport-sync.conf and the sample
• Removes systemd unit files from /lib/systemd/system/,
  /etc/systemd/system/ and /usr/local/lib/systemd/system/
• Removes the shairport-sync user and group
• Removes UFW rules added during install (5353/udp, 319/udp, 320/udp,
  7000/tcp)
• Backs up the current config under /tmp/airplay_uninstall_backup_<ts>/
  before deleting anything
• Verifies the removal and reports leftover items, if any

APT build dependencies are intentionally not removed (they may be
in use by other software); the script prints the apt-get command to
remove them manually for users who want a fully clean state.

New: airplay_manager.sh

Thin wrapper that exposes a single menu:

1. Install AirPlay 2 → calls install_airplay_v3.sh
2. Modify existing installation → calls modify_airplay.sh
3. Uninstall → calls uninstall_airplay.sh
4. Tail live service logs

It auto-detects whether shairport-sync is installed and grays out
modify/uninstall when no installation is present.

README.md

Rewritten to document the new scripts, the built-in audio support, the
Trixie/libplist-utils note, and a troubleshooting section with the
most common failure modes.

Compatibility

• All scripts refuse to run as root and invoke sudo only where
  needed — same convention as the original installer.
• No changes to systemd unit content, NQPTP, or the
  shairport-sync --with-airplay-2 build flags.
• Existing installations done with the old version of
  install_airplay_v3.sh are fully compatible with the new
  modify_airplay.sh / uninstall_airplay.sh.

Test plan

• bash -n passes on all four scripts.
• Pi 4 (Trixie) with only built-in audio: install succeeds, card
  appears in picker, audio plays from 3.5mm jack.
• Pi 4 with USB DAC only: card auto-selected, AirPlay endpoint
  visible on iPhone.
• Pi 4 with both built-in and USB DAC: both shown in picker, either
  choice works.
• modify_airplay.sh: rename device → confirm new name appears in
  iPhone AirPlay picker after restart.
• modify_airplay.sh: switch from DAC to built-in (and back) →
  output_device, mixer_control_name, mixer_device updated
  coherently; service restarts cleanly.
• uninstall_airplay.sh: full removal on a freshly installed Pi
  leaves no leftover items in the verification step; reinstalling
  afterwards works.
• airplay_manager.sh: menu reflects the correct state before/after
  install and uninstall.
• Trixie: install no longer fails at the plistutil check.