Let the Device field be picked, not typed blind

Diagnosing capture on a real TV meant reading pactlSources off the
screen and typing an exact PulseAudio source name back in through the
same remote-driven text field — no way to copy-paste, easy to
mistype, and the one piece of information (which source, if any, is
actually RUNNING) was buried in a JSON dump.

Added two choice() pickers bound to the same capture.device setting:
one built from pactlSources (pulse/auto backends), one built from
alsaCapturePcms (alsa backend), both parsed from diagnostics the
service already collects — no new Luna method needed. Diagnostics
already run once at boot, so the picker is populated immediately,
before the user ever presses "Run diagnostics" by hand.

Picking a value writes straight into capture.device, and the plain
text field stays as the fallback for anything the parser misses.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -116,6 +116,9 @@ native/        the webOS service: capture, DSP, sinks, Luna API (C)
 frontend/      the on-TV app (plain HTML/CSS/JS, no framework)
 servicefiles/  services.json, package.json and the boot script
 host/          the receiver and loopback setup for the HyperHDR machine
+docker/        the receiver, packaged as a container (e.g. for Unraid)
+unraid/        the plugin for the one part a container can't do: the
+               ALSA loopback kernel module, persisted across reboots
 tools/         build, packaging, asset generation, on-TV probe
 test/          host-side tests: wire formats, the capture pipeline, the UI
 docs/          the longer explanations