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/src/main.rs

@@ -0,0 +1,10 @@
+// Desktop binary entry point. All logic lives in `lib.rs` so the same
+// code can be built as a cdylib for Android/iOS via `cargo tauri android build`.
+#![cfg_attr(
+    all(not(debug_assertions), target_os = "windows"),
+    windows_subsystem = "windows"
+)]
+
+fn main() {
+    wzp_desktop_lib::run();
+}