refactor(client): extract UI-agnostic plumbing into pf-client-core (phase 0)

Session pump, FFmpeg decode, PipeWire audio, SDL3 gamepads, keymap, trust
store, mDNS discovery, library client and Wake-on-LAN move verbatim from
clients/linux into crates/pf-client-core, shared with the upcoming Vulkan
session binary (punktfunk-planning: linux-client-rearchitecture.md).
The GTK client re-exports them at the crate root so every existing
crate::-path keeps resolving; its manifest drops the moved-only deps.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-tray/Cargo.toml

@@ -23,7 +23,7 @@ anyhow = "1"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 # Loopback HTTPS poll of GET /api/v1/local/summary. Same sync ureq + rustls(ring) stack and
-# custom-verifier pattern as the Linux client's library fetch (clients/linux/src/library.rs) —
+# custom-verifier pattern as the Linux client's library fetch (crates/pf-client-core/src/library.rs) —
 # but ring-only (no default aws-lc-rs provider: it needs a C toolchain per target and the agent
 # pins the ring provider explicitly anyway).
 ureq = { version = "2", default-features = false, features = ["tls"] }