feat: add @ opening to the mention ring

Cut a clean gap in the lower-right of the circle for the @ opening, with no
separate tail curve.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GXfHYcViokBQRtqCW9vLQi

amethyst/src/main/res/drawable/ic_notif_mention.xml

@@ -4,7 +4,7 @@
     android:viewportWidth="24"
     android:viewportHeight="24"
     android:tint="#000000">
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+  <path android:fillColor="#000000" android:pathData="M16.037,19.593L15.36,19.916L14.658,20.179L13.935,20.38L13.197,20.516L12.45,20.588L11.7,20.595L10.952,20.536L10.212,20.412L9.486,20.224L8.778,19.974L8.096,19.663L7.443,19.293L6.824,18.868L6.245,18.391L5.71,17.865L5.223,17.295L4.787,16.684L4.407,16.037L4.084,15.36L3.821,14.658L3.62,13.935L3.484,13.197L3.412,12.45L3.405,11.7L3.464,10.952L3.588,10.212L3.776,9.486L4.026,8.778L4.337,8.096L4.707,7.443L5.132,6.824L5.609,6.245L6.135,5.71L6.705,5.223L7.316,4.787L7.963,4.407L8.64,4.084L9.342,3.821L10.065,3.62L10.803,3.484L11.55,3.412L12.3,3.405L13.048,3.464L13.788,3.588L14.514,3.776L15.222,4.026L15.904,4.337L16.557,4.707L17.176,5.132L17.755,5.609L18.29,6.135L18.777,6.705L19.213,7.316L19.593,7.963L19.916,8.64L20.179,9.342L20.38,10.065L20.516,10.803L20.588,11.55L20.595,12.3L20.536,13.048L20.412,13.788L20.224,14.514L19.974,15.222L18.119,14.472L18.312,13.93L18.456,13.372L18.551,12.804L18.596,12.23L18.591,11.655L18.536,11.081L18.431,10.515L18.277,9.96L18.075,9.421L17.827,8.901L17.535,8.405L17.201,7.937L16.827,7.499L16.416,7.095L15.972,6.729L15.497,6.403L14.996,6.119L14.472,5.881L13.93,5.688L13.372,5.544L12.804,5.449L12.23,5.404L11.655,5.409L11.081,5.464L10.515,5.569L9.96,5.723L9.421,5.925L8.901,6.173L8.405,6.465L7.937,6.799L7.499,7.173L7.095,7.584L6.729,8.028L6.403,8.503L6.119,9.004L5.881,9.528L5.688,10.07L5.544,10.628L5.449,11.196L5.404,11.77L5.409,12.345L5.464,12.919L5.569,13.485L5.723,14.04L5.925,14.579L6.173,15.099L6.465,15.595L6.799,16.063L7.173,16.501L7.584,16.905L8.028,17.271L8.503,17.597L9.004,17.881L9.528,18.119L10.07,18.312L10.628,18.456L11.196,18.551L11.77,18.596L12.345,18.591L12.919,18.536L13.485,18.431L14.04,18.277L14.579,18.075L15.099,17.827Z"/>
   <group
       android:scaleX="0.03"
       android:scaleY="0.03"