feat(sound): loop a "waiting to connect" cue during SSH agent approval

[Nick6489]

What

Adds a looping "Waiting to connect" sound cue that plays while an SFTP
connection is stalled waiting for the SSH agent to approve the key.

An external SSH agent (Bitwarden's agent, a smartcard) shows that approval in a
dialog that, for a screen-reader user, tends to open behind the Portkey Drop
window with nothing to announce it. The connection then sits there with no
feedback. This fills that gap.

How it works

• AgentAuthNotice (portkeydrop-core::protocols) -- a new optional
  Arc<dyn Fn()> hook on SftpClient / create_client. SFTP auth calls it once
  on the connect worker, immediately before the first authenticate_publickey_with
  agent signature request -- the exact point an external agent can prompt.
• The UI's connect worker wraps it in a events::notify_awaiting_agent call,
  which posts AppEvent::ConnectAwaitingAgent.
• MainFrame::handle_event logs and announces "Waiting for SSH key approval"
  and calls AppState::start_waiting_sound(), which loops the connect_waiting
  cue.
• Connected and ConnectFailed call stop_waiting_sound(); so does
  disconnect_quietly() (top of every connect, and disconnect) and on_close().

Looping playback (new in the sound layer)

play_looping_sound_file decodes the file once into a SamplesBuffer, repeats
it, and returns a LoopHandle. Dropping the handle -- or calling .stop() --
sets a flag the playback source checks per sample, so the loop ends within one
audio buffer. SoundPlayer::play_event_looping is the pack-aware wrapper.

Looping cues are not counted by wait_for_playback, so the exit-sound wait
never blocks on one.

Sound event

connect_waiting is registered in sound_events.rs under Connections, so it
shows in Settings' per-event mute list and is validated like any other cue. No
audio is bundled for it (the built-in pack ships no audio at all in this repo);
a pack opts in with a connections/connect_waiting.ogg entry. Until then the
event still logs and announces -- just silently.

Tests

• player.rs: StopFlag ends the source when the flag is set and reports its
  real channel layout; LoopHandle stops on stop() and on drop; a missing or
  undecodable file yields no handle; volume 0 yields an inert handle without
  touching the device.
• events.rs: notify_awaiting_agent posts ConnectAwaitingAgent without
  waiting for a reply.
• state.rs: start_waiting_sound / stop_waiting_sound are a safe no-op in
  the silent (headless / test) state.
• sound_events.rs: connect_waiting is a known event.

cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings,
and cargo test --workspace all pass. The Linux (Ubuntu) clippy path for the
cfg(unix) agent code could not be run locally (no WSL distro); the change to
that function is a new parameter used only in shared code.

Independence from #147

Branched from dev, not from #147. Both touch soundpacks/player.rs and
CHANGELOG.md, so whichever merges second needs a small conflict resolve in the
use block / changelog list -- they are otherwise unrelated and can merge in
either order.

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