videowall: audio mixer beneath the canvas, audio/video/both source types, microphone source, and live VU meters

[Copilot]

The videowall switcher kept the audio mixer in a separate tab, treated every source as video, had no audio-input source, couldn't show a full 1920×1080 canvas without scrolling, and its VU meters never moved.

Changes

• Mixer under the canvas with a splitter — draw_stage() stacks the canvas tabs over the Audio Mixer with a draggable horizontal splitter (height persisted in AppState::mixer_height, clamped so neither pane collapses). The mixer is no longer a tab.
• Fit-to-view canvas — draw_editable_canvas() scales by min(scale_w, scale_h) and centers the canvas, replacing the old width-only scale in a fixed scrolling box, so the whole wall/1080p bus is visible at once.
• Source media type — new MediaType { Video, Audio, Both } per source, defaulted per kind (default_media_type) and overridable in the prepare form. source_has_video / source_has_audio route placement: audio → mixer, video → canvas, both → both. Canvas drops reject audio-only; mixer skips video-only.
• Microphone source — new SourceKind::Microphone (audio-only) that enumerates and connects an audio capture device via pulse_device_session_connect_device, with matching teardown and no video pipeline.
• VU meters — the mapping was already correct (matches pexninja); meters read silence because no source captured audio. A Microphone source now feeds pulse_register_device_audio_level_callback (PULSE_MEDIA_INPUT) real input, so its meter moves. Sources with no captured audio still read silence, as expected.

Notes

The VU answer is intrinsic to the API: PULSE_MEDIA_INPUT audio levels reflect a captured input device, not decoded stream audio — so RTMP/RTSP/MP4/conference strips marked Both will read flat even though they carry audio. Routing and the mixer strip still make sense for them; only the live-meter signal is mic-specific.

enum class MediaType { Video = 0, Audio, Both };

static bool source_has_video(const ActiveSource & s)
{ return s.media_type == MediaType::Video || s.media_type == MediaType::Both; }
static bool source_has_audio(const ActiveSource & s)
{ return s.media_type == MediaType::Audio || s.media_type == MediaType::Both; }