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/Notifications/NotificationState.swift

@@ -0,0 +1,30 @@
+import Foundation
+
+/// Persisted "what we've already notified about", so the background poller
+/// only fires a notification when something actually changes:
+/// - a PR that wasn't there before → "new PR"
+/// - a PR whose CI aggregate status changed → "CI status changed"
+struct NotificationState: Codable {
+    /// PR ids (repo.fullName#number) we've already announced as new.
+    var announcedNewPRs: Set<String> = []
+    /// PR id → last notified CI status string ("success"/"failure"/"running"/etc).
+    var ciStatusByPR: [String: String] = [:]
+
+    private static let url: URL = {
+        let dir = FileManager.default.urls(for: .cachesDirectory, in: .userDomainMask)[0]
+        return dir.appendingPathComponent("notification-state.json")
+    }()
+
+    func save() {
+        guard let data = try? JSONEncoder().encode(self) else { return }
+        try? data.write(to: Self.url)
+    }
+
+    static func load() -> NotificationState {
+        guard let data = try? Data(contentsOf: url),
+              let state = try? JSONDecoder().decode(NotificationState.self, from: data) else {
+            return NotificationState()
+        }
+        return state
+    }
+}