feat(release): m6 release mechanics — signed APK, R8, version guard, icons

Release-hardening pass (PLAN.md §9 M6, §10, §12). No architecture, data-flow,
or endpoint changes; the app remains a pure API client.

App icons (default brand assets):
- New gateway-medallion launcher icon set (all densities, adaptive fg/bg, round,
  Play Store icon) + an RG notification icon staged for M7 push.
- Replace the Image Asset wizard's default green-grid adaptive background with the
  deep-indigo brand fill (@color/ic_launcher_background #1B1033); recomposite the
  legacy square/round webps and the 512 Play icon over indigo so the whole set is
  coherent (the green never shipped). Restore the SPDX headers the wizard stripped;
  drop the orphaned placeholder foreground vector. No <monochrome> layer — the
  full-colour medallion has no clean silhouette, so themed mode falls back to the
  standard icon rather than a tinted blob.

Version-mismatch guard (§3):
- The connect probe now refuses a Runic Gateway backend whose API version this
  build can't speak (e.g. a future v2) with a clear "app out of date" message,
  instead of mis-rendering; lenient on a blank api (older backend). Decision logic
  extracted to a pure ConnectionRepository.evaluateVersion() with unit tests.

Release build hardening (§7, §12):
- Enable R8 full-mode minify + resource shrink for release (~31 MB debug -> 4.2 MB
  signed release). ProGuard keep-rules for kotlinx.serialization serializers + our
  wire DTOs, Retrofit service interfaces, and a -dontwarn for Tink's compile-only
  Error Prone annotations (EncryptedSharedPreferences).
- Release signingConfig reads keystore material from a gitignored keystore.properties
  or env vars; absent -> unsigned (debug + PR gate unaffected). Keystore never in repo.
- versionName/versionCode overridable via -P so the release tag + CI run number
  drive them (§10).

CI:
- release.yml: on a `v*` tag, build a SIGNED release APK (keystore from a base64
  Gitea secret) and attach it + SHA256SUMS to a Gitea release; workflow_dispatch is
  a signing dry run. Mirrors pr-checks.yml's self-hosted-runner handling (apt JDK 17,
  explicit sdkmanager, in-step chmod +x gradlew).

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

app/proguard-rules.pro

@@ -1,3 +1,59 @@
-# Runic Gateway Android app — ProGuard/R8 rules.
-# Minification is disabled until M6 (release hardening); real keep rules for
-# kotlinx.serialization DTOs and Retrofit models are added there.
+# Runic Gateway Android app — ProGuard/R8 rules (release minify + resource shrink, M6).
+#
+# The dependency stack ships its own consumer rules that R8 applies automatically:
+# Retrofit 2.11, OkHttp 4.12, kotlinx.serialization 1.7 (core), Hilt/Dagger, Coil 2.7.
+# The rules below are defensive belt-and-suspenders for the areas full-mode R8 is
+# most likely to over-strip in this app: the kotlinx.serialization generated
+# serializers and our own @Serializable wire DTOs.
+
+# ── kotlinx.serialization (canonical keep rules) ────────────────────────────
+-keepattributes *Annotation*, InnerClasses
+-dontnote kotlinx.serialization.**
+
+# Keep the Companion of @Serializable classes so `.serializer()` resolves.
+-if @kotlinx.serialization.Serializable class **
+-keepclassmembers class <1> {
+    static <1>$Companion Companion;
+}
+-if @kotlinx.serialization.Serializable class ** {
+    static **$Companion Companion;
+}
+-keepclassmembers class <2>$Companion {
+    kotlinx.serialization.KSerializer serializer(...);
+}
+# Keep `INSTANCE.serializer()` of @Serializable objects.
+-if @kotlinx.serialization.Serializable class ** {
+    public static ** INSTANCE;
+}
+-keepclassmembers class <1> {
+    public static <1> INSTANCE;
+    kotlinx.serialization.KSerializer serializer(...);
+}
+# Keep the synthesized $$serializer classes and their descriptor field.
+-keepclassmembers class **$$serializer {
+    *** descriptor;
+}
+
+# ── Our wire DTOs ───────────────────────────────────────────────────────────
+# All request/response models decoded by kotlinx.serialization. Keeping them
+# (and their generated serializers) guarantees additive backend fields and
+# @SerialName mappings survive minification. DTOs are small, so keeping them
+# whole is cheap insurance against a full-mode strip.
+-keep @kotlinx.serialization.Serializable class com.runicgateway.app.** { *; }
+-keepclassmembers class com.runicgateway.app.data.api.dto.** { *; }
+
+# ── Retrofit service interfaces ─────────────────────────────────────────────
+# Retrofit reads method + parameter annotations reflectively; keep our API
+# interfaces' generic signatures so return types (suspend .../Call<T>) resolve.
+-keep,allowobfuscation interface com.runicgateway.app.data.api.*Api
+-keepattributes Signature, Exceptions
+
+# Kotlin metadata is needed for reflection over Kotlin types (serialization/Retrofit).
+-keep class kotlin.Metadata { *; }
+
+# ── Tink / EncryptedSharedPreferences (androidx.security-crypto) ─────────────
+# Tink references Error Prone compile-only annotations that are absent at runtime;
+# they are safe to ignore (they carry no runtime behaviour). Suppresses the R8
+# "Missing class com.google.errorprone.annotations.*" errors.
+-dontwarn com.google.errorprone.annotations.**
+