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  build can't speak (e.g. a future v2) with a clear "app out of date" message,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/src/test/java/com/runicgateway/app/data/repository/ConnectionVersionGuardTest.kt

@@ -0,0 +1,51 @@
+/*
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+package com.runicgateway.app.data.repository
+
+import com.runicgateway.app.data.api.dto.VersionDto
+import com.runicgateway.app.data.repository.ConnectionRepository.Companion.evaluateVersion
+import com.runicgateway.app.data.repository.ConnectionRepository.VersionVerdict
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+/**
+ * The connect-probe version guard (§3): a Runic Gateway backend on the app's
+ * supported API is accepted; a wrong service identity or an unsupported API
+ * version is refused clearly rather than mis-rendered.
+ */
+class ConnectionVersionGuardTest {
+
+    @Test
+    fun `matching service and api is Ok`() {
+        val v = VersionDto(service = "runic-gateway", api = "v1", server = "1.2.3")
+        assertEquals(VersionVerdict.Ok, evaluateVersion(v))
+    }
+
+    @Test
+    fun `service id is case-insensitive and trimmed`() {
+        val v = VersionDto(service = "  Runic-Gateway  ", api = "V1", server = "x")
+        assertEquals(VersionVerdict.Ok, evaluateVersion(v))
+    }
+
+    @Test
+    fun `wrong service is NotRunicGateway`() {
+        val v = VersionDto(service = "some-other-app", api = "v1", server = "x")
+        assertEquals(VersionVerdict.NotRunicGateway, evaluateVersion(v))
+    }
+
+    @Test
+    fun `blank api is lenient (older backend) and accepted`() {
+        val v = VersionDto(service = "runic-gateway", api = "", server = "x")
+        assertEquals(VersionVerdict.Ok, evaluateVersion(v))
+    }
+
+    @Test
+    fun `future api version is a mismatch carrying the server value`() {
+        val v = VersionDto(service = "runic-gateway", api = "v2", server = "x")
+        val verdict = evaluateVersion(v)
+        assertTrue(verdict is VersionVerdict.Mismatch)
+        assertEquals("v2", (verdict as VersionVerdict.Mismatch).serverApi)
+    }
+}