feat(gotify): Phase 4 C1 — WebSocket reconnect + 帧解析

- src/gotify/reconnect.rs: ReconnectPolicy 按 §15 退避序列
  1s/2s/4s/8s/16s/30s...record_success() 重置；inline #[cfg(test)]
  3 个 backoff 测试
- src/gotify/websocket.rs: blocking tungstenite 0.21 客户端
  · stream_url() http→ws / https→wss 自动转
  · build_request() 手拼 Sec-WebSocket-* header + Authorization: Bearer
  · connect_once() 单次 handshake
  · run_stream_loop() 永久循环：connect → read Message::Text 解析
    GotifyMessage → 回调 on_message；Ping 触发 Pong；Close/断开/读
    错都走到 §15 退避后重连
- src/gotify/mod.rs: re-export 加 reconnect + websocket 子模块
- Cargo.toml: tungstenite 0.21 (rustls-tls-webpki-roots + handshake
  feature) + http 1 + url 2
- tests/gotify_websocket.rs: 3 个 JSON envelope 解析测试

避坑：
- tungstenite 0.21 的 connect 在 handshake feature 后（默认不带），要
  显式开
- WebSocketConfig 0.21 没 connect_timeout 字段；靠 rustls 默认 ~30s
  上限（Phase 4 LAN 部署可接受；Phase 8 再补 socket-level timeout）
- read_message() 在 0.21 已 deprecated；改用 read()
- Message 枚举加覆盖 Pong / Frame 两支（compiler non-exhaustive 警告）
- inline #[cfg(test)] mod tests 在 reconnect.rs 跑 unit tests（与
  tests/ 集成测试并列）

验证：cargo build / build --release 都 exit 0，0 warning
- cargo test: 12 passed (3 reconnect unit + 6 api + 3 websocket); 0 failed
- debug   33.36 MB, subsystem = WINDOWS_CUI
- release  9.43 MB, subsystem = WINDOWS_GUI（tungstenite + http 在 LTO
  后几乎不增体积——和 Phase 3 C2 持平）

Cargo.toml

@@ -44,6 +44,11 @@ reqwest = { version = "0.12", default-features = false, features = ["blocking",
 # --- Date/time (Phase 3 C2 convert Gotify RFC3339) ---
 chrono = { version = "0.4", default-features = false, features = ["clock", "serde"] }
 
+# --- WebSocket (Phase 4 real-time stream) ---
+tungstenite = { version = "0.21", default-features = false, features = ["rustls-tls-webpki-roots", "handshake"] }
+url         = "2"
+http        = "1"
+
 # Slint's `.slint` files are compiled by `build.rs`; see build.rs.
 [build-dependencies]
 slint-build = "1"