sync: the client header said "desktop" from every phone, and named the wrong version

`client_headers()` built `thoughtsync-desktop/{CARGO_PKG_VERSION}`, and both
halves were wrong.

This crate is compiled into the Android app as well as the desktop one, so
every phone in the field announced itself as a desktop. And CARGO_PKG_VERSION
here is the CORE crate's version — a number no build stamps and no user has
ever seen — where the thing a reader of that header wants is the app's own
build (note 3127 §5: with no version tags, the artifact's self-report is the
only answer to "which build is this?").

The core cannot know either value, so the host says them. `set_client_agent`
is a OnceLock the desktop fills in `run()` and Android fills in
`ThoughtSyncApplication.onCreate`, before anything can sync. A host that never
introduces itself sends `thoughtsync-unidentified/unknown` rather than a
plausible default: nothing reads this header today, which is exactly why a
wrong value could sit in it for months — the first person to look at a server
log is the first who could catch it, and only if what they see is obviously a
host that never said who it was.

Android's version comes from the INSTALLED package, through a new
`Context.installedVersionName()` that the foot of the Sync screen now shares.
One answer to "which build is on this phone", so the line a person quotes in a
bug report and the line in the server's log cannot disagree.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K3MMqUtzX1TJgA1oypvm1c

android/ffi/src/lib.rs

@@ -520,6 +520,20 @@ pub fn checklist_render(text: String, checked: bool) -> String {
     local::derive::render_item(&text, checked)
 }
 
+/// Tell the core which app it is running inside, and which build of it.
+///
+/// Android has to say so because the core cannot: the same crate is compiled into
+/// the desktop app, and it used to announce every phone in the field as
+/// `thoughtsync-desktop` carrying the CORE crate's version — a number no build
+/// stamps and nobody has seen. The honest value is the installed package's own
+/// `versionName`, which is what Kotlin passes here.
+///
+/// Called once from `ThoughtSyncApplication.onCreate`, before anything can sync.
+#[uniffi::export]
+pub fn set_client_agent(name: String, version: String) {
+    compat::set_client_agent(&name, &version);
+}
+
 /// Every checklist item in a body, with the line each one sits on — so a renderer
 /// walking the body line by line knows which lines are boxes and what is in them.
 #[uniffi::export]