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>

frontend/js/mock.js

@@ -173,7 +173,14 @@
             },
             binaries: { parec: false, pactl: true, pacat: false, arecord: true },
             pulseSockets: ['/var/run/pulse/native'],
-            pactlSources: 'mock output',
+            // Realistic shape: a TV that mixes several per-app sinks down to
+            // one common output, the case the device picker exists for.
+            pactlSources: [
+              '0\ttpcm_output.monitor\tmodule-combine-sink.c\ts16le 2ch 48000Hz\tRUNNING',
+              '1\ttpmedia.monitor\tmodule-alsa-card.c\ts16le 2ch 48000Hz\tIDLE',
+              '2\ttpeffects.monitor\tmodule-alsa-card.c\ts16le 2ch 48000Hz\tIDLE',
+              '3\ttptts.monitor\tmodule-alsa-card.c\ts16le 2ch 48000Hz\tSUSPENDED',
+            ].join('\n'),
             alsaCards: ['0 [Loopback]: Loopback - Loopback'],
             alsaCapturePcms: ['00-01: Loopback PCM : playback 1 : capture 1'],
           },