version: every surface can say which build it is, and two of them were lying

Note 3127 §5 removed version tags, so an artifact's self-report is now the only
answer to "which build is this?" — and nothing exists to contradict it when it
is wrong. Three surfaces gain a dim build line: the foot of the web rail, the
login screen, and the foot of Sync on Android.

The login screen because "I can't sign in" is a bug report like any other, and
requiring an account to read a build number withholds it from exactly the people
who can't get past that page. `/api/config` is already public.

Two of the values it was going to show were wrong, which is the part worth
knowing about.

The DESKTOP reported `env!("CARGO_PKG_VERSION")` from `config_get` and from the
startup log. `cargo tauri build --config '{"version": ...}'` overrides
tauri.conf.json, not Cargo's own metadata — so both read the literal `0.2.0` in
Cargo.toml, on every build ever shipped. They now read a display version baked in
by the lane through `option_env!`, hoisted to the crate root because two readers
of one fact is how this repo keeps producing 2181-2183. Not the ordering key
either: `1.0.<minutes>` is the opaque value Tauri's updater compares and must
never be shown to a person, and `update.rs` still reads it because a comparator
is exactly what it is (rule 149).

The SERVER fell back to `__version__` when APP_VERSION was absent, so a server
run from a checkout reported `0.2.0` — a real-looking version naming no build
anybody could obtain. `__init__.py` already asserted the honest answer was
"APP_VERSION being missing, which app.py already handles"; it did not, and a
comment claiming a behaviour two files away is how that stayed true-sounding.
Now an explicit "unknown", with the packaging version left where "unknown" is
not a legal value.

Android reads the INSTALLED package's versionName rather than BuildConfig, so it
reports what is actually on the phone.

Everything renders "unknown" rather than blank when it cannot say. A blank looks
like a layout bug; a plausible default cannot be caught by anything.

build.rs gets `rerun-if-env-changed` for the baked value: cargo does not track an
`option_env!` variable on its own, and the desktop lane having no cache today is
what makes that easy to forget the day one is added.

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

@@ -31,6 +31,7 @@ import androidx.compose.runtime.remember
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.res.pluralStringResource
 import androidx.compose.ui.res.stringResource
 import androidx.compose.ui.text.font.FontWeight
@@ -120,10 +121,43 @@ fun SyncScreen(
                         onDismissRevokeNotice = onDismissRevokeNotice,
                     )
             }
+
+            BuildLine()
         }
     }
 }
 
+/**
+ * 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.
+ */
+@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
+        }
+    Text(
+        text = version,
+        style = MaterialTheme.typography.bodySmall,
+        color = MaterialTheme.colorScheme.onSurfaceVariant,
+        modifier = Modifier.padding(bottom = 16.dp),
+    )
+}
+
 // ─────────────────────────────────  linked  ─────────────────────────────────
 
 @Composable

android/app/src/main/res/values/strings.xml

@@ -200,4 +200,9 @@
 
     <!-- Errors -->
     <string name="error_dismiss">Dismiss</string>
+
+    <!-- The build, at the foot of Sync. Never blank: an APK with no versionName is
+         a real state (a bare `gradlew assembleDebug` with no override) and saying
+         so is better than an empty line that reads as a layout bug. -->
+    <string name="build_unknown">unknown</string>
 </resources>