feat(soundpack): add the connect_waiting cue audio

[Nick6489]

Adds connect_waiting.ogg — the audio for the connect_waiting sound event
introduced in #148 (looping "waiting to connect" cue during SSH-agent approval).

48 kHz stereo Ogg Vorbis, 3s, built to loop. Mine, contributed under the
project's licence.

Heads-up: the default sound pack has no audio in this repo

Nothing here loads this file yet, on purpose — but it surfaced something worth
fixing.

• PackManifest::default_pack_json() is { "name": "Default", ..., "sounds": {} }
  — an empty stub.
• ensure_default_pack() only writes that manifest; it copies no audio.
• The Windows build's "Stage the install tree" step copies PortkeyDrop.exe,
  prism.dll, and version.txt into dist/PortkeyDrop_dir/ and nothing else,
  and the installer ships that tree.

So a fresh install — installer or nightly — gets a default pack that plays
nothing. Existing users only have cues because the ~/.portkeydrop → platform
config-dir migration carried the Python-era pack across.

What I'd suggest

Vendor the default sounds into the repo (this file, plus the ~19 you already
shipped with the Python version) and point default_pack_json() at them, so a
brand-new user — including nightly testers — hears cues on first launch
without touching a thing.

crates/portkeydrop-core/assets/soundpacks/default/ is just where I put this
one; move it wherever fits.

Landmines when you wire it up:

• manifest.rs::the_generated_default_manifest_is_valid_and_empty and the
  assert_eq!(text, PackManifest::default_pack_json()) check in soundpacks/mod.rs
  both assert the stub stays empty.
• should_write_default_manifest() treats a non-empty manifest as the
  user's and won't overwrite it — a populated default_pack_json() changes that
  "safe to regenerate" logic.
• ensure_default_pack() writes the manifest but nothing puts the audio on
  disk, so the files need an include_dir! + write-on-first-run, or a copy step
  in build.yml and the installer.

Merge order

Independent of #148 at the file level (this PR only adds a binary), but the file
is only useful once #148's connect_waiting event exists. Either order is
fine.

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