fix(windows): grant microphone and camera permissions in WebView2

WebView2 silently denies getUserMedia() unless a PermissionRequested
handler explicitly allows it. macOS was already covered by Info.plist;
Windows had nothing. Adds a COM event handler via with_webview that
auto-approves mic and camera requests so Element Call voice/video
works in the desktop app.

Also includes previously uncommitted changes:
- tauri.conf.json: add media-src / mediastream: to CSP
- Info.plist: macOS NSMicrophoneUsageDescription / NSCameraUsageDescription

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src-tauri/Info.plist

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+    <key>NSCameraUsageDescription</key>
+    <string>Request camera access for WebRTC calls.</string>
+    <key>NSMicrophoneUsageDescription</key>
+    <string>Request microphone access for WebRTC calls.</string>
+</dict>
+</plist>