Add app icon (mic + waveform) and fix desktop categories

- share/icons/hicolor/: SVG source + PNG fallbacks (48-256 px); the
  desktop entry now uses Icon=meetrec instead of a stock theme icon
- install.sh installs the icon into the hicolor tree, refreshes the
  GTK icon cache, and cleans up on uninstall
- Categories fixed to AudioVideo;Audio per the menu spec (bare Audio
  is a subcategory requiring its main category); desktop-file-validate
  is clean now

share/applications/meetrec.desktop

@@ -5,8 +5,8 @@ Name=MeetRec
 GenericName=Meeting Recorder
 Comment=Record in-person meetings and transcribe them with Whisper
 Exec=meetrec
-Icon=audio-microphone
+Icon=meetrec
 Terminal=false
-Categories=Audio;
+Categories=AudioVideo;Audio;
 Keywords=meeting;recording;recorder;transcription;transcript;whisper;audio;microphone;
 StartupNotify=true