fix(linux): stop fighting the A2DP profile, and make the codec choosable

[injkgz]

Problem

On Linux, pressing play on the AirPods stem did not resume playback reliably. Watching the MPRIS traffic showed the play command arriving and being cancelled right after:

kded6       -> Play      (stem press)
wireplumber -> Pause     (~0.5s later)
kded6       -> Play
wireplumber -> Pause

The cause is in MediaController: it re-activates its cached A2DP profile on every playback start. Switching the card profile recreates the PipeWire sink, which stops the stream that triggered the activation, and WirePlumber pauses the MPRIS player.

While tracing that, two more problems in the same path showed up, both matching existing issues: the cached card index goes stale across reconnects, and a not-yet-enumerated card triggers an immediate WirePlumber restart that silences audio for the better part of a minute.

Changes

fix(linux): stop fighting the A2DP profile on playback start — closes #680, #681, #682

• An already-active a2dp-sink* profile is left untouched; the preferred profile is selected only when the card is off or on a non-A2DP profile. The negotiated codec is preserved.
• The card is resolved by Bluetooth MAC before every activation and the cached index refreshed, so a changed index after a reconnect is picked up without restarting the app.
• Profile enumeration gets a bounded grace period (5 × 1s) before a WirePlumber restart is considered. The restart stays as a last resort, retries after it are bounded, and no sleep happens after the final failed attempt. A persistent failure is logged and returns without activating a profile.

feat(linux): let the user pick the preferred A2DP codec

The preference order was hardcoded as ["a2dp-sink-sbc_xq", "a2dp-sink-sbc", "a2dp-sink"], so AAC was never selected — even though it is what AirPods use natively with Apple devices and what PipeWire ranks highest for them. A Preferred audio codec setting (AAC / SBC-XQ / SBC) now lives in the settings tab, is stored as preferred_codec in app_settings.json, and defaults to AAC. The remaining codecs are kept as fallbacks.

Testing

Tested on Fedora 44 (KDE Plasma, Wayland), PipeWire 1.6.8, WirePlumber 0.5.14, BlueZ 5.87, with AirPods Pro 3 (model A3064):

• Stem press pause/resume, repeated: only kded6 sends MPRIS commands now, no cancelling Pause follows, and the card stays on a2dp-sink throughout.
• Playback start with A2DP already active logs A2DP profile a2dp-sink already active, leaving it unchanged and performs no profile switch.
• Disconnect/reconnect cycles (case closed/opened): no WirePlumber restarts were triggered, and audio came back on its own.
• HFP switching for calls still works: the profile moves to headset-head-unit while a client captures from the AirPods microphone and returns to a2dp-sink afterwards.
• Codec setting: switching between AAC, SBC-XQ and SBC in the settings tab takes effect on the next activation.

cargo build --release is clean; no new warnings.