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/app/src/main/java/com/fabledsword/thoughtsync/BuildIdentity.kt

@@ -0,0 +1,21 @@
+package com.fabledsword.thoughtsync
+
+import android.content.Context
+
+/**
+ * The `versionName` of the INSTALLED package, or null when it cannot be read.
+ *
+ * From the package manager rather than from `BuildConfig`: this reports what is
+ * actually on the phone, which is the question both callers are asking — a bug
+ * report reading the foot of Sync, and a server log reading the client header. It
+ * also needs no `buildFeatures.buildConfig`, which this module does not enable.
+ *
+ * Returns null rather than a fallback string, because the two callers want
+ * different ones: the UI wants a localized "unknown" from string resources, the
+ * client header wants the literal the core recognizes. Note 3127 §5 governs both —
+ * with no version tags, the artifact's self-report is the only answer to "which
+ * build is this?", so a missing name must read as missing and never as a plausible
+ * default that nothing can contradict.
+ */
+fun Context.installedVersionName(): String? =
+    runCatching { packageManager.getPackageInfo(packageName, 0).versionName }.getOrNull()

android/app/src/main/java/com/fabledsword/thoughtsync/ThoughtSyncApplication.kt

@@ -3,6 +3,7 @@ package com.fabledsword.thoughtsync
 import android.app.Application
 import android.util.Log
 import com.fabledsword.thoughtsync.core.ThoughtSync
+import com.fabledsword.thoughtsync.core.setClientAgent
 
 /**
  * Opens the shared Rust core once, for the process lifetime.
@@ -30,6 +31,14 @@ class ThoughtSyncApplication : Application() {
 
     override fun onCreate() {
         super.onCreate()
+
+        // Introduce this app to any server it links to, before anything can sync.
+        // The core cannot name us — the same crate is compiled into the desktop app,
+        // and it used to announce every phone as `thoughtsync-desktop` carrying the
+        // core crate's own version. "unknown" rather than a guess when the package
+        // manager will not say (note 3127 §5).
+        setClientAgent("thoughtsync-android", installedVersionName() ?: "unknown")
+
         try {
             val handle = ThoughtSync(filesDir.absolutePath)
             core = handle

android/app/src/main/java/com/fabledsword/thoughtsync/ui/SyncScreen.kt

@@ -38,6 +38,7 @@ import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.unit.dp
 import com.fabledsword.thoughtsync.R
 import com.fabledsword.thoughtsync.UpdateOutcome
+import com.fabledsword.thoughtsync.installedVersionName
 
 /**
  * Opt-in server pairing.
@@ -131,25 +132,20 @@ fun SyncScreen(
  * The build, dim, at the foot of Sync — the same thing the web UI puts at the
  * bottom of its rail (#3181).
  *
- * Read from the INSTALLED package rather than from `BuildConfig`: this reports what
- * is actually on the phone, which is the question a bug report is asking. It also
- * needs no `buildFeatures.buildConfig`, which this module does not enable.
- *
  * Note 3127 §5 is why it is here at all. With version tags gone, an artifact's own
  * self-report is the only answer to "which build is this?" — so it renders
  * "unknown" rather than nothing when the name is absent, because a blank line looks
  * like a layout bug and a plausible default cannot be caught by anything.
+ *
+ * The read itself is `installedVersionName()`, shared with the client header the
+ * app sends its server: 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.
  */
 @Composable
 private fun BuildLine() {
     val context = LocalContext.current
     val unknown = stringResource(R.string.build_unknown)
-    val version =
-        remember(context) {
-            runCatching {
-                context.packageManager.getPackageInfo(context.packageName, 0).versionName
-            }.getOrNull() ?: unknown
-        }
+    val version = remember(context) { context.installedVersionName() ?: unknown }
     Text(
         text = version,
         style = MaterialTheme.typography.bodySmall,