Speed up landing screen: drop per-PR enrichment + add inbox cache; bump to 2.1

The inbox was slow because it enriched every PR with CI runs + reviews (2
extra API calls per PR, sequential). Drop that from the list — CI/review
status now shows only in the PR detail screen. Add a disk cache so the landing
screen renders instantly from the last load, then refreshes in the background
and keeps the cached list on network failure. Bump to 2.1.

GiteaPR/Inbox/InboxCache.swift

@@ -0,0 +1,43 @@
+import Foundation
+
+/// Persists the inbox PR list to disk so the landing screen can render
+/// instantly from the last load, then refresh in the background.
+struct InboxCache {
+    private let fileURL: URL
+
+    init() {
+        let dir = FileManager.default.urls(for: .cachesDirectory,
+                                           in: .userDomainMask)[0]
+        fileURL = dir.appendingPathComponent("inbox.json")
+    }
+
+    func load() -> [PRRow] {
+        guard let data = try? Data(contentsOf: fileURL) else { return [] }
+        return (try? Self.decoder.decode([PRRow].self, from: data)) ?? []
+    }
+
+    func save(_ rows: [PRRow]) {
+        guard let data = try? JSONEncoder().encode(rows) else { return }
+        try? data.write(to: fileURL)
+    }
+
+    /// Same ISO8601-with-offset date strategy the API decoder uses, so the
+    /// cached `Date?` fields decode correctly.
+    private static let decoder: JSONDecoder = {
+        let d = JSONDecoder()
+        let formatter = ISO8601DateFormatter()
+        formatter.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
+        d.dateDecodingStrategy = .custom { decoder in
+            let container = try decoder.singleValueContainer()
+            let str = try container.decode(String.self)
+            if let date = formatter.date(from: str) { return date }
+            let plain = ISO8601DateFormatter()
+            plain.formatOptions = [.withInternetDateTime]
+            if let date = plain.date(from: str) { return date }
+            throw DecodingError.dataCorruptedError(
+                in: container,
+                debugDescription: "Cannot decode date string \(str)")
+        }
+        return d
+    }()
+}