feat(icons): swap post + incognito_off drawables for Material Symbols

- BookmarkGroupItem: replace R.drawable.post with MaterialSymbols.News
  (codepoint U+E032). Drop now-unused post.xml.
- IncognitoBadge: drop the NIP-17 "encrypted" indicator (encryption is the
  expected default; no need to flag) and replace the legacy NIP-04
  unencrypted indicator with MaterialSymbols.NoEncryption (U+F03F).
  Drop now-unused incognito_off.xml.

incognito.xml is kept — still used by the "post anonymously" toggle in
ToggleNip17Button, GenericCommentPostScreen, and ShortNotePostScreen.

https://claude.ai/code/session_016gV9gEaSud6kDMdcaqy8ti

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/icons/symbols/MaterialSymbols.kt

@@ -142,6 +142,8 @@ object MaterialSymbols {
     val MoreVert = MaterialSymbol("\uE5D4")
     val MoveToInbox = MaterialSymbol("\uE168")
     val MusicNote = MaterialSymbol("\uE405")
+    val News = MaterialSymbol("\uE032")
+    val NoEncryption = MaterialSymbol("\uF03F")
     val Notifications = MaterialSymbol("\uE7F5")
     val Numbers = MaterialSymbol("\uEAC7")
     val OpenInBrowser = MaterialSymbol("\uE89D")