Add push-equivalent notifications via background polling; bump to 3.6

Sideloaded apps can't use APNs (no push entitlement without Apple signing),
so deliver notifications via an iOS BGAppRefreshTask that polls the monitored
repos every ~15 min and fires local notifications (no Apple push needed):
- new PRs that appear in the open set
- CI status changes on existing PRs

Persistence: NotificationState tracks announced PR ids + last-known CI status
so we only fire on actual changes, not every poll. AppDelegate registers the
background task and reschedules the next one. Info.plist gets UIBackgroundModes
fetch + the BGTask identifier. Settings gains a Notifications toggle; also
poll after a foreground inbox load so state seeds and changes are caught
promptly. Bump to 3.6.

GiteaPR/Settings/SettingsStore.swift

@@ -36,6 +36,11 @@ final class SettingsStore: ObservableObject {
     /// Set when a PR is merged, so the inbox knows to reload on next appearance
     /// (otherwise a just-merged PR could linger as stale in the list).
     @Published var inboxNeedsRefresh = false
+    /// Whether background polling for push-equivalent notifications is on.
+    @Published var notificationsEnabled: Bool {
+        didSet { UserDefaults.standard.set(notificationsEnabled,
+                                           forKey: "notificationsEnabled") }
+    }
 
     private init() {
         instanceURL = KeychainHelper.get("instanceURL") ?? "https://gitea.bueso.eu"
@@ -44,6 +49,7 @@ final class SettingsStore: ObservableObject {
         mergeGateMode = MergeGateMode(rawValue: raw ?? "") ?? .warn
         deleteBranchAfterMerge = UserDefaults.standard.bool(forKey: "deleteBranchAfterMerge")
         monitoredRepos = UserDefaults.standard.stringArray(forKey: "monitoredRepos") ?? []
+        notificationsEnabled = UserDefaults.standard.object(forKey: "notificationsEnabled") as? Bool ?? false
     }
 
     var isConfigured: Bool {