feat(android,ios): offer posters and synopses on the library, as the desktop does

The offer appears when metadata is off and there is a library to put
posters on. It says what turning metadata on sends, links to Settings
and never turns anything on itself. "Not now" is stored on the device.
The existing library screenshots now run with metadata on, as their
shelf implies, and a new screenshot covers the offer.

ios/ProtonStream/App/ProtonStreamApp.swift

@@ -203,7 +203,8 @@ struct RootView: View {
             onAddShare: {
                 incomingLink = ""
                 destination = .shares
-            }
+            },
+            onOpenSettings: { select(.settings) }
         )
         if width >= twoPaneWidth {
             HStack(spacing: 0) {

ios/ProtonStream/App/SettingsStore.swift

@@ -51,6 +51,12 @@ struct SettingsStore {
         nonmutating set { defaults.set(String(describing: newValue), forKey: Key.librarySort) }
     }
 
+    /// The library's offer of posters and synopses was turned down.
+    var metadataOfferDismissed: Bool {
+        get { bool(Key.metadataOfferDismissed, default: false) }
+        nonmutating set { defaults.set(newValue, forKey: Key.metadataOfferDismissed) }
+    }
+
     /// Whether the library folds each franchise into one tile.
     var libraryGrouped: Bool {
         get { bool(Key.libraryGrouped, default: true) }
@@ -70,5 +76,6 @@ struct SettingsStore {
         static let cacheBudgetGib = "cache_budget_gib"
         static let librarySort = "library_sort"
         static let libraryGrouped = "library_grouped"
+        static let metadataOfferDismissed = "metadata_offer_dismissed"
     }
 }