Surface self-updates through the regular updates list

Fold Aurora Store's self-update back into the existing update pipeline
instead of a dedicated sheet/viewmodel/helper. The feed entry is mapped
to a regular App and added to the update list, reusing the standard
download + install path; it is never auto-installed silently.

- Add a dedicated "Self-update" section in the Updates tab with the
  app-details navigation disabled (it's served from the Aurora OSS feed).
- Gate eligibility via PackageUtil.isSelfUpdateSupported(): vanilla/preload
  flavors, not debug, not F-Droid (huawei excluded by flavor).
- Add a build-aware Settings toggle as the opt-out, removing the row
  immediately when disabled.
- Exempt nightly self-updates from deleteInvalidUpdates (static version
  code) and drop any stale self-update row when none is offered, so a
  phantom update doesn't linger after a self-install.

app/build.gradle.kts

@@ -23,6 +23,10 @@ val lastCommitHash = providers.exec {
     commandLine("git", "rev-parse", "--short", "HEAD")
 }.standardOutput.asText.map { it.trim() }
 
+val lastCommitTimestamp = providers.exec {
+    commandLine("git", "log", "-1", "--format=%ct")
+}.standardOutput.asText.map { it.trim() }
+
 java {
     toolchain {
         languageVersion = JavaLanguageVersion.of(21)
@@ -69,6 +73,7 @@ configure<ApplicationExtension> {
         testInstrumentationRunnerArguments["disableAnalytics"] = "true"
 
         buildConfigField("String", "EXODUS_API_KEY", "\"bbe6ebae4ad45a9cbacb17d69739799b8df2c7ae\"")
+        buildConfigField("long", "BUILD_TIMESTAMP", "${lastCommitTimestamp.get()}L")
 
         missingDimensionStrategy("device", "vanilla")
     }