icon: Inkwell's mark is a black inkpot and quill on the brand yellow

Step 5 of milestone 481. Replaces the linked-notes constellation, which had been
stale since note links were dropped (alembic 0024). The colour scheme stays.

packaging/icons.py draws the mark once and renders every variant from it: the
rounded tile (web, desktop), the maskable full-bleed web icon, and the Android
adaptive foreground. The detail (shaft, vane splits, glint) is cut out of the ink
with a mask rather than painted on in yellow, because the Android foreground is
now transparent and its alpha is also the themed-icon silhouette. The old
foreground was the opaque maskable tile, which a themed icon would have drawn as
a solid square.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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

@@ -3,10 +3,12 @@
   Adaptive icon. minSdk is 26, so this is the ONLY icon Android will ask for —
   no legacy raster fallback is needed.
 
-  The foreground is the shared maskable asset the web app already ships
-  (frontend/public/icon-maskable-512.png), which is drawn with the safe-zone
-  padding adaptive icons require. Reusing it means the phone, the web app and the
-  desktop all wear the same face rather than three near-misses.
+  The foreground is the inkwell mark alone on transparency, inside the 66dp safe
+  circle; the yellow is the background colour. It is rendered by
+  packaging/icons.py from the same drawing as the web and desktop icons, so all
+  three wear one face. Transparent rather than a full-bleed tile because the
+  monochrome layer below reuses it: a themed icon draws its alpha as a silhouette,
+  and an opaque foreground would come out as a solid square.
 -->
 <adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
     <background android:drawable="@color/ic_launcher_background" />