Credit Material Symbols, and state the assistant's terms

Two gaps in the licence file, both from work done after it was written.

The share, lookup and assistant glyphs are Material Symbols path data copied
into our own vector files. The Libraries table credits the Material icons
library, which is not the same thing as copying its paths, so the drawables now
have their own entry and appear in the app's About page.

The assistant was not mentioned at all. It ships no vendor SDK, no key and no
default endpoint, so nothing non-free is distributed — but a reader may point
it at a service whose terms are their own, and what a model writes back belongs
to neither Ganjoor nor this app. Written down for F-Droid's sake as much as the
reader's.

No new dependencies came with any of it: the assistant reuses the HTTP client
and JSON parser already here.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/src/main/java/com/ganjoor/android/ui/AboutScreen.kt

@@ -137,6 +137,14 @@ private val CREDITS = listOf(
         "apache-2.0.txt",
         "https://google.github.io/accompanist/",
     ),
+    Credit(
+        "Material Symbols",
+        "The Android Open Source Project — the share, lookup and assistant glyphs, drawn from " +
+            "their path data rather than pulled in as a library",
+        "Apache License 2.0",
+        "apache-2.0.txt",
+        "https://fonts.google.com/icons",
+    ),
     Credit(
         "Ganjoor for Android",
         "This app",