feat(desktop): split main.rs into lib.rs for Tauri Mobile (Android/iOS)

Tauri 2.x Mobile links the app as a cdylib loaded from a Java Activity, so
all of the Builder/command code has to live in a library crate. Move the
existing logic verbatim into src/lib.rs::run() and reduce src/main.rs to a
two-line desktop entry point that calls into it.

Cargo.toml gets a [lib] section (crate-types: staticlib + cdylib + rlib,
named wzp_desktop_lib) and the wzp-client dependency — which pulls CPAL +
VoiceProcessingIO — is moved behind cfg(not(target_os = "android")) so the
Android cdylib doesn't need an audio backend yet. Engine-backed Tauri
commands (connect/disconnect/toggle_mic/toggle_speaker/get_status) get
Android stubs that return clear "not yet wired" errors. The signaling
commands (register_signal/place_call/answer_call/get_signal_status/
ping_relay/get_identity) are platform-independent and unchanged.

Also: get_identity / register_signal now auto-create the seed if missing
instead of erroring with "connect to a room first", and the identity dir
resolves to /data/data/com.wzp.phone/files/.wzp on Android (proper
app-internal storage) vs \$HOME/.wzp on desktop.

Side note: src/main.rs was previously untracked — desktop builds were
working only because it existed in the local worktree. This commit fixes
that too.

Step 1 of the Android rewrite plan (tauri-mobile scaffold). No audio yet.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/src-tauri/Cargo.toml

@@ -5,6 +5,16 @@ edition = "2024"
 description = "WarzonePhone Desktop — encrypted VoIP client"
 default-run = "wzp-desktop"
 
+# Library target — required for Tauri mobile (Android/iOS link the app as a cdylib)
+# and also used by the desktop binary below.
+[lib]
+name = "wzp_desktop_lib"
+crate-type = ["staticlib", "cdylib", "rlib"]
+
+[[bin]]
+name = "wzp-desktop"
+path = "src/main.rs"
+
 [build-dependencies]
 tauri-build = { version = "2", features = [] }
 
@@ -19,12 +29,16 @@ tracing-subscriber = "0.3"
 anyhow = "1"
 rustls = { version = "0.23", default-features = false, features = ["ring", "std"] }
 
-# WarzonePhone crates
+# WarzonePhone crates — protocol layer is platform-independent
 wzp-proto = { path = "../../crates/wzp-proto" }
 wzp-codec = { path = "../../crates/wzp-codec" }
 wzp-fec = { path = "../../crates/wzp-fec" }
 wzp-crypto = { path = "../../crates/wzp-crypto" }
 wzp-transport = { path = "../../crates/wzp-transport" }
+
+# wzp-client pulls CPAL + (on macOS) VoiceProcessingIO — desktop only.
+# Android gets an oboe/AAudio backend in Step 3 of the Tauri mobile rewrite.
+[target.'cfg(not(target_os = "android"))'.dependencies]
 wzp-client = { path = "../../crates/wzp-client", features = ["audio", "vpio"] }
 
 # Platform-specific