chore(scaffold): M0 Gradle + Compose + Hilt skeleton with CI

Stand up the Android-app repo per docs/android/PLAN.md M0: a buildable
Kotlin + Jetpack Compose (Material 3) single-activity skeleton wired for
Hilt, ready for the M1-M4 functional pass.

- Gradle 8.7 wrapper; AGP 8.6.1 / Kotlin 2.0.20, JDK 17, minSdk 29, target 35.
- Version catalog (gradle/libs.versions.toml) pins the full planned stack
  (Compose, Hilt, Retrofit/OkHttp + kotlinx.serialization, DataStore,
  security-crypto, Coil, Navigation) so later milestones reference by alias.
- RunicGatewayApp (@HiltAndroidApp) + MainActivity (Compose) + ui/theme/*.
- Strings externalized from day one; adaptive launcher icon; backup rules
  exclude the token store / DataStore (no session material off-device).
- CI: .gitea/workflows/pr-checks.yml gates PRs with lint + test + assembleDebug
  (JDK 17 + Android SDK on the self-hosted runner; debug builds auto-signed,
  no secrets). Placeholder JVM unit test so the test gate runs.
- .gitattributes forces LF on gradlew so the wrapper runs on the Linux runner.

Verified locally: `gradle help`/`projects` configure the :app module and
resolve all six plugins cleanly (full assemble needs the Android SDK, done in CI).

app id: com.runicgateway.app (PLAN.md §13, pending runicgateway.app domain).

Co-Authored-By: Claude <noreply@anthropic.com>

app/src/main/res/drawable/ic_launcher_foreground.xml

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- SPDX-License-Identifier: GPL-3.0-or-later -->
+<!-- Placeholder launcher glyph: a stylized gateway arch. Replaced in the M5 design pass. -->
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    android:width="108dp"
+    android:height="108dp"
+    android:viewportWidth="108"
+    android:viewportHeight="108">
+    <path
+        android:fillColor="#D0BCFF"
+        android:pathData="M38,74 L38,50 A16,16 0 0,1 70,50 L70,74 L62,74 L62,50 A8,8 0 0,0 46,50 L46,74 Z" />
+    <path
+        android:fillColor="#EFB8C8"
+        android:pathData="M52,34 L56,34 L56,40 L52,40 Z M50,40 L58,40 L58,44 L50,44 Z" />
+</vector>