feat(storage): Phase 5 C1 — AppConfig 持久化到 config.json

- src/storage/config.rs: AppConfig + ServerEntry + load_config +
  save_config，atomic write（tmp + rename + fsync）；多 server
  数据结构按 §21 落地但 V1 只 active 一个；3 个 #[cfg(test)]
  round-trip + default + missing-field 测试
- src/storage/state.rs: 从 src/storage.rs 拆过来；atomic write + 2
  个测试（default 960×680 + JSON round-trip）
- src/storage/mod.rs: pub use re-export 两组 API
- src/config.rs: 重构为 ActiveConfig thin wrapper，调
  storage::load_config() 拿到第一个 selected_server；placeholder
  hardcoded URL/token 在 §19/§20 意义上不再合适，删掉
- src/gotify/model.rs: ServerConfig 加 Serialize/Deserialize（要
  写到 config.json 必须能 serialize；token 仍走 §20 不进日志）
- src/lib.rs: run() 启动用 ActiveConfig::load() 取代 placeholder；
  on_test_connection_requested 同样改用 ActiveConfig::load()
- 删除 src/storage.rs（被 src/storage/{config,state}.rs 替代；
  同样内容已迁过去）

避坑：
- ServerConfig 之前没 derive Serialize，所以 #[serde(flatten)]
  嵌套到 ServerEntry 失败——加 derive 是必要的
- src/storage.rs 和 src/storage/mod.rs 不能并存，否则 Rust 报
  ambiguous module；删旧文件就行
- AppConfig 多 vec + serde default 让旧 config 文件（字段缺失）
  也能 forward-compat parse（3 个测试覆盖）

验证：cargo build / build --release 都 exit 0，0 warning
- cargo test: 14 passed (5 storage unit + 6 api + 3 websocket); 0 failed
- debug   35.54 MB, subsystem = WINDOWS_CUI
- release  9.76 MB, subsystem = WINDOWS_GUI

src/config.rs

@@ -1,29 +1,66 @@
-//! Application configuration placeholder.
+//! Active-config view used at runtime.
 //!
-//! Phase 3 ships only a hardcoded `placeholder()` so the UI wiring can
-//! demonstrate the real-data path. Phase 5 replaces this with persistent
-//! JSON storage at `%APPDATA%/WindowsGotify/config.json` (AGENTS.md §5/§19/§20).
-//!
-//! AGENTS.md §20 — the placeholder token below is intentionally not a
-//! real credential. Real auth fails, the UI falls back to mock data, and
-//! nothing about the token ever reaches a log line.
+//! Phase 5 C1: this is a thin wrapper over `storage::AppConfig`. It
+//! surfaces the single selected server as `ServerConfig` so the rest
+//! of the runtime (`gotify::GotifyApi`, `gotify::websocket`) doesn't
+//! have to care about the multi-server storage layout. The placeholder
+//! URL/token that lived here in Phase 3 C2 is gone — first-run users
+//! will see "no server configured" until they fill in Settings
+//! (Phase 6 hooks up the Settings → Connection form).
 
+use crate::error::AppResult;
 use crate::gotify::ServerConfig;
+use crate::storage::AppConfig as PersistedConfig;
 
-pub struct AppConfig {
+pub struct ActiveConfig {
     pub server: Option<ServerConfig>,
+    pub appearance: String,
+    pub desktop_notifications: bool,
+    pub sound: bool,
+    pub notify_high: bool,
+    pub notify_max: bool,
+    pub start_with_windows: bool,
+    pub start_minimized: bool,
+    pub close_to_tray: bool,
 }
 
-impl AppConfig {
-    /// Hardcoded single-server placeholder. Returns `None` here means
-    /// "no server configured" — but for now Phase 3 always returns `Some`
-    /// so the UI wiring is exercised on every start.
-    pub fn placeholder() -> Self {
+impl ActiveConfig {
+    /// Load from `%APPDATA%/WindowsGotify/config.json` (or defaults on
+    /// missing / malformed file). Returns `Ok(ActiveConfig)` even when
+    /// there's no server configured — the caller decides how to
+    /// surface that in the UI.
+    pub fn load() -> AppResult<Self> {
+        let persisted = crate::storage::load_config();
+        Ok(Self::from_persisted(persisted))
+    }
+
+    fn from_persisted(p: PersistedConfig) -> Self {
+        let server = p
+            .active_server()
+            .or_else(|| p.first_enabled_server())
+            .map(|entry| entry.config.clone());
         Self {
-            server: Some(ServerConfig::new(
-                "http://192.168.50.103:50334",
-                "placeholder-token-do-not-log",
-            )),
+            server,
+            appearance: p.appearance,
+            desktop_notifications: p.desktop_notifications,
+            sound: p.sound,
+            notify_high: p.notify_high,
+            notify_max: p.notify_max,
+            start_with_windows: p.start_with_windows,
+            start_minimized: p.start_minimized,
+            close_to_tray: p.close_to_tray,
         }
     }
+
+    /// Build an `ActiveConfig` from a fully-formed `PersistedConfig`
+    /// value (handy for tests and for the upcoming Settings → Save
+    /// path, which constructs the persisted config in memory and then
+    /// asks us to mirror it back).
+    pub fn from_persisted_value(p: PersistedConfig) -> Self {
+        Self::from_persisted(p)
+    }
+
+    pub fn placeholder() -> Self {
+        Self::from_persisted(PersistedConfig::default())
+    }
 }