Implement iOS share extension queue flow and progress UI

[futurepaul]

Summary

• add iOS PikaShareExtension target and shared queue storage (ShareQueueManager) in app-group container
• sync auth/chat cache from AppManager into share extension state and process queued shares on foreground/deep-link
• implement native share extension UI for text/url/image payloads with recent chat picker
• keep send queue-based (no Rust core in extension), add optional compose text, and show staged progress (Preparing -> Sending To Queue -> Queued) before dismiss
• make simulator iOS builds code-signed by default so app/extension entitlements and app-group sharing work reliably

Why

• avoids DB/runtime lock contention by not instantiating FfiApp inside the extension
• matches expected share-sheet UX with in-extension confirmation and no forced app switch on successful queue
• preserves fallback Open Pika path for sign-in/recovery

Testing

• ./tools/xcode-run xcodebuild -project ios/Pika.xcodeproj -target PikaShareExtension -configuration Debug -sdk iphonesimulator build (pass)
• ./tools/run-ios --sim (pass, signed simulator build, app launches)
• attempted focused xcodebuild test for PikaTests/AppManagerTests; build currently fails in this environment due xcframework simulator slice mismatch, unrelated to share-extension Swift changes