iPhone app: declare photo saving so Save Image appears; record what was tested

The share sheet only offers Save Image when the app declares
NSPhotoLibraryAddUsageDescription; without it screenshots couldn't be saved to
Photos. docs/iphone.md now lists what was verified against the Frame (Bonjour
discovery, waiting and reconnecting, upload, share sheet, install links,
opening Steam Link) and the two permission prompts iOS shows.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

ios/FrameControl/Info.plist

@@ -49,6 +49,8 @@
 	</array>
 	<key>NSLocalNetworkUsageDescription</key>
 	<string>Frame Control finds your Steam Frame on your network and connects to it.</string>
+	<key>NSPhotoLibraryAddUsageDescription</key>
+	<string>Frame Control saves headset captures and screenshots to your photo library when you ask it to.</string>
 	<key>UILaunchScreen</key>
 	<dict>
 		<key>UIColorName</key>