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

install.sh

@@ -58,6 +58,7 @@ if [ "$UNINSTALL" = 1 ]; then
     rm -rf "$APP_DIR"
     rm -f "$BIN_DIR/meetrec" "$BIN_DIR/meetrec-cli"
     rm -f "$DESKTOP_DIR/meetrec.desktop"
+    find "$PREFIX/share/icons/hicolor" -name 'meetrec.*' -type f -delete 2>/dev/null || true
     if command -v update-desktop-database >/dev/null 2>&1; then
         update-desktop-database "$DESKTOP_DIR" 2>/dev/null || true
     fi
@@ -150,12 +151,20 @@ if [ -x "$PREFIX/share/meetrec/whisper-cpp/bin/whisper-cli" ]; then
     echo "    (whisper.cpp engine installed — select 'whisper-cpp' to use the GPU)"
 fi
 
-echo "==> Installing XDG desktop entry"
+echo "==> Installing XDG desktop entry and icon"
 install -m 0644 "$SCRIPT_DIR/share/applications/meetrec.desktop" \
     "$DESKTOP_DIR/meetrec.desktop"
+for f in "$SCRIPT_DIR"/share/icons/hicolor/*/apps/meetrec.* \
+         "$SCRIPT_DIR"/share/icons/hicolor/scalable/apps/meetrec.svg; do
+    rel="${f#"$SCRIPT_DIR"/share/icons/}"
+    install -Dm 0644 "$f" "$PREFIX/share/icons/$rel"
+done
 if command -v update-desktop-database >/dev/null 2>&1; then
     update-desktop-database "$DESKTOP_DIR" 2>/dev/null || true
 fi
+if command -v gtk-update-icon-cache >/dev/null 2>&1; then
+    gtk-update-icon-cache -q -t -f "$PREFIX/share/icons/hicolor" 2>/dev/null || true
+fi
 
 if [ "$DOWNLOAD_MODEL" = 1 ]; then
     echo "==> Pre-downloading Whisper model '$MODEL' (first run only, can take a while)"