Add app icons across web, Android, and the desktop app

- Web: favicon.ico + png icons under static/, served from the site root
  and linked from every page head.
- Android: the artwork becomes the launcher icon — the 432px adaptive
  foreground over the gruvbox background, replacing the placeholder vector
  drawables.
- Desktop: the icon is the Qt window/taskbar icon (embedded via app.qrc),
  and the app is now an installable package — a wrapped binary, a
  photog.desktop entry, and hicolor icons — so `nix profile install
  ./desktop` puts "Photog" in the application menu.

android/app/src/main/res/drawable/ic_launcher_background.xml

@@ -1,10 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<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="#1d2021"
-        android:pathData="M0,0h108v108h-108z" />
-</vector>

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

@@ -1,28 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<!-- Aperture blades in Gruvbox aqua on the adaptive-icon 108dp canvas.
-     The safe zone is the middle 72dp, so the art is inset accordingly. -->
-<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="#8ec07c"
-        android:pathData="M54,30 A24,24 0 0,1 78,54 L54,54 Z" />
-    <path
-        android:fillColor="#689d6a"
-        android:pathData="M78,54 A24,24 0 0,1 54,78 L54,54 Z" />
-    <path
-        android:fillColor="#8ec07c"
-        android:pathData="M54,78 A24,24 0 0,1 30,54 L54,54 Z" />
-    <path
-        android:fillColor="#689d6a"
-        android:pathData="M30,54 A24,24 0 0,1 54,30 L54,54 Z" />
-
-    <path
-        android:strokeColor="#ebdbb2"
-        android:strokeWidth="3"
-        android:fillColor="#00000000"
-        android:pathData="M54,28 A26,26 0 1,1 53.99,28 Z" />
-</vector>

android/app/src/main/res/mipmap-anydpi-v26/ic_launcher.xml

@@ -1,7 +1,9 @@
 <?xml version="1.0" encoding="utf-8"?>
 <!-- minSdk is 28, so every device resolves the v26 adaptive icon and no
-     density-bucket PNG fallbacks are needed. -->
+     density-bucket PNG fallbacks are needed. The foreground is the full circular
+     app icon (ic_launcher_foreground.png); the launcher mask crops the disc to
+     its shape, and the solid gruvbox background fills the transparent corners. -->
 <adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
-    <background android:drawable="@drawable/ic_launcher_background" />
-    <foreground android:drawable="@drawable/ic_launcher_foreground" />
+    <background android:drawable="@color/gruvbox_bg" />
+    <foreground android:drawable="@mipmap/ic_launcher_foreground" />
 </adaptive-icon>

android/app/src/main/res/mipmap-anydpi-v26/ic_launcher_round.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
 <adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
-    <background android:drawable="@drawable/ic_launcher_background" />
-    <foreground android:drawable="@drawable/ic_launcher_foreground" />
+    <background android:drawable="@color/gruvbox_bg" />
+    <foreground android:drawable="@mipmap/ic_launcher_foreground" />
 </adaptive-icon>