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

README.md

@@ -44,7 +44,7 @@ This:
 1. copies the app to `~/.local/share/meetrec`,
 2. creates a venv and installs all dependencies,
 3. installs the `meetrec` (GUI) and `meetrec-cli` launchers into `~/.local/bin`,
-4. installs the XDG desktop entry (`~/.local/share/applications/meetrec.desktop`),
+4. installs the XDG desktop entry and icon (app menu shows **MeetRec** with its own red mic icon),
 5. pre-downloads the `small` Whisper model (first run is otherwise slow).
 
 Options:
@@ -139,6 +139,7 @@ install.sh            install / uninstall into a local prefix
 bin/meetrec           GUI launcher (installed into ~/.local/bin)
 bin/meetrec-cli       CLI launcher (installed into ~/.local/bin)
 share/applications/meetrec.desktop   XDG desktop entry
+share/icons/hicolor/                app icon (SVG + PNG fallbacks)
 android/              native Android app (Kotlin + whisper.cpp JNI)
 ```