feat(android): add monochrome themed launcher icon (#4733) (#4736)

Android 13+ recolors the adaptive icon's `<monochrome>` layer with a
wallpaper-derived tint when the user enables themed icons. Support was added
in #2122/#2153 (the `ic_launcher_monochrome.png` assets) but #2353 ("fixed
launcher icon size") rewrote the committed adaptive icon to inset the
foreground 22% and silently dropped the `<monochrome>` layer, so themed icons
stopped working in shipped builds.

Restore it by re-adding a `<monochrome>` layer (same 22% inset as the
foreground) and shipping the monochrome mipmaps. The CI/release flow
regenerates `gen/android` (`tauri android init` + `tauri icon`) then
`git checkout .` to restore tracked customizations; `tauri icon` does not emit
a monochrome layer, so the mipmaps are force-committed under `gen/` like the
other customized resources.

The monochrome artwork is redesigned: Android tints the layer via SRC_IN
(alpha only), which flattened the old desaturated-logo asset into a solid
blob. A narrow vertical center gap now splits the open book into two pages
with a visible spine while keeping the bookmark, so the mark keeps its
character when themed.

Verified on a Pixel 9 Pro emulator (Android 36) with Themed Icons enabled, and
guarded by src/__tests__/android/themed-icon.test.ts.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/readest-app/src-tauri/gen/android/app/src/main/res/mipmap-anydpi-v26/ic_launcher.xml

@@ -6,4 +6,9 @@
       android:drawable="@mipmap/ic_launcher_foreground"
       android:inset="22%"/>
   </foreground>
+  <monochrome>
+    <inset
+      android:drawable="@mipmap/ic_launcher_monochrome"
+      android:inset="22%"/>
+  </monochrome>
 </adaptive-icon>

apps/readest-app/src/__tests__/android/themed-icon.test.ts

@@ -0,0 +1,49 @@
+import { describe, it, expect } from 'vitest';
+import { existsSync, readFileSync } from 'fs';
+import { resolve } from 'path';
+
+/**
+ * Regression guard for the Android themed ("Material You" / monochrome) launcher
+ * icon (issue #4733).
+ *
+ * Android 13+ recolors the adaptive icon's `<monochrome>` layer with a
+ * wallpaper-derived tint when the user enables themed icons. Support for it was
+ * originally added in #2122/#2153 (the `ic_launcher_monochrome.png` assets) and
+ * wired into the source adaptive icon, but #2353 ("fixed Android launcher icon
+ * size") rewrote the *committed* `gen/` adaptive icon to inset the foreground
+ * and silently dropped the `<monochrome>` layer — so themed icons stopped
+ * working in shipped builds.
+ *
+ * The CI/release flow regenerates `gen/android` from scratch
+ * (`tauri android init` + `tauri icon`) and then `git checkout .` to restore the
+ * *tracked* customizations. `tauri icon` does not emit a monochrome layer, so
+ * the themed icon only survives if both the adaptive-icon XML and the
+ * monochrome mipmaps are tracked under `gen/`.
+ *
+ * Invariants:
+ *  1. The committed adaptive icon declares a `<monochrome>` layer pointing at
+ *     `@mipmap/ic_launcher_monochrome`.
+ *  2. A tracked `ic_launcher_monochrome.png` exists for every launcher density
+ *     so the resource resolves at build time (these are force-added past the
+ *     `gen/` .gitignore, like the other customized resources).
+ */
+
+const appRoot = process.cwd();
+const resRoot = resolve(appRoot, 'src-tauri/gen/android/app/src/main/res');
+
+const DENSITIES = ['mdpi', 'hdpi', 'xhdpi', 'xxhdpi', 'xxxhdpi'];
+
+describe('Android themed (monochrome) launcher icon', () => {
+  it('declares a <monochrome> layer in the adaptive icon', () => {
+    const xml = readFileSync(resolve(resRoot, 'mipmap-anydpi-v26/ic_launcher.xml'), 'utf8');
+    expect(xml).toMatch(/<monochrome\b/);
+    expect(xml).toMatch(/@mipmap\/ic_launcher_monochrome/);
+  });
+
+  it('ships a tracked monochrome mipmap for every density', () => {
+    const missing = DENSITIES.filter(
+      (d) => !existsSync(resolve(resRoot, `mipmap-${d}`, 'ic_launcher_monochrome.png')),
+    );
+    expect(missing, `missing tracked monochrome mipmaps for: ${missing.join(', ')}`).toEqual([]);
+  });
+});