fix: use BufferSize::Default for PipeWire ALSA devices on Linux

[nightwatch92]

cpal's BufferSize::Fixed calls snd_pcm_hw_params_set_buffer_size (exact),
which pipewire-alsa rejects unless the frame count matches its current
quantum. Fall back to BufferSize::Default (uses _near) for default/pulse/
pipewire device names on Linux.

Also emit a targeted error when pactl is not installed instead of the
generic 'no monitor input found' message — the previous message sent
users chasing ALSA config when the real fix is installing pulseaudio-utils.

Fixes #2131.

Greptile Summary

This PR improves Linux audio capture compatibility and diagnostics.

• Uses CPAL’s default buffer-size negotiation for Linux devices named default, pulse, or pipewire.
• Reports targeted installation guidance when system-audio discovery cannot find pactl on PATH.

Confidence Score: 5/5

The PR appears safe to merge, with no concrete changed-code defect identified.

The buffer-selection change is Linux-scoped and narrowly targets the device aliases described by the fix, while the new diagnostic preserves successful monitor selection and only changes the terminal error path.

Important Files Changed

Filename	Overview
crates/recording/src/feeds/microphone.rs	Selects BufferSize::Default for recognized Linux PulseAudio/PipeWire ALSA device names while preserving fixed latency buffers elsewhere.
crates/recording/src/sources/screen_capture/linux.rs	Adds a dedicated missing-pactl diagnostic after existing CPAL and pactl monitor-source discovery attempts fail.

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