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

share/icons/hicolor/scalable/apps/meetrec.svg

@@ -0,0 +1,26 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg width="128" height="128" viewBox="0 0 128 128" xmlns="http://www.w3.org/2000/svg">
+  <title>MeetRec</title>
+  <defs>
+    <linearGradient id="bg" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0" stop-color="#e74c3c"/>
+      <stop offset="1" stop-color="#922b21"/>
+    </linearGradient>
+  </defs>
+
+  <rect x="4" y="4" width="120" height="120" rx="28" fill="url(#bg)"/>
+
+  <!-- microphone: capsule + cradle -->
+  <rect x="50" y="18" width="28" height="42" rx="14" fill="#ffffff"/>
+  <path d="M 36 48 v 8 a 28 28 0 0 0 56 0 v -8"
+        fill="none" stroke="#ffffff" stroke-width="9" stroke-linecap="round"/>
+
+  <!-- waveform: voice becoming transcript -->
+  <g fill="#ffffff">
+    <rect x="32" y="96" width="8" height="14" rx="4"/>
+    <rect x="46" y="91" width="8" height="24" rx="4"/>
+    <rect x="60" y="95" width="8" height="16" rx="4"/>
+    <rect x="74" y="91" width="8" height="24" rx="4"/>
+    <rect x="88" y="96" width="8" height="14" rx="4"/>
+  </g>
+</svg>