Replace pulsectl-rs with local pulse module (#241)

* Replace pulsectl-rs with local pulse module

pulsectl-rs is archived and has unfixed bugs: SourceController uses
sink (not source) APIs for get_default_device, set_device_mute_by_index,
and set_device_mute_by_name, causing --input-volume mute-toggle to
silently fail.

Replace it with a focused pulse module (~350 lines) that calls
libpulse-binding directly with the correct sink/source APIs for each
controller. This also removes the substring dependency.

* Use a global Pulse Context instead of recreating it every call

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Co-authored-by: Erik Reider <35975961+ErikReider@users.noreply.github.com>