Replace the default Flutter icon with a stylised cloud

The app shipped the Flutter template's blue F, which said nothing about what it
does and did not match the interface. The launcher icon is now a white cloud on
the same seed blue the app themes itself with, so the two read as one product.

The icon is drawn rather than hand-edited: tool/generate_icon.py renders it from
signed distance fields — a union of three circles and a rounded box — using only
the standard library, so regenerating it needs no image toolchain. The shape's
framing is derived from its own bounding box, which is what keeps it centred;
maintaining the bounds separately from the shape had it drifting off-centre.

Adds the adaptive icon and the Android 13 monochrome layer, which were missing
entirely. The generator frames the cloud against the 72dp the launcher mask
always keeps and adaptive_icon_foreground_inset is 0, so the layer is not shrunk
twice and the icon reads at the same size as its neighbours on a home screen.

Verified on the emulator: the cloud appears in the app drawer and holds its shape
down to 48px. docs/architecture.md records how to regenerate it, including the
flutter_launcher_icons 0.14.4 bug that writes a string into a boolean Xcode
setting and needs reverting by hand afterwards.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="utf-8"?>
+<adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
+  <background android:drawable="@color/ic_launcher_background"/>
+  <foreground>
+      <inset
+          android:drawable="@drawable/ic_launcher_foreground"
+          android:inset="0%" />
+  </foreground>
+  <monochrome>
+      <inset
+          android:drawable="@drawable/ic_launcher_monochrome"
+          android:inset="0%" />
+  </monochrome>
+</adaptive-icon>

Nuvolari/app/android/app/src/main/res/values/colors.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <color name="ic_launcher_background">#1F6FB2</color>
+</resources>