feat: v0.3.0 - fix provider data loss, oauth corruption & switch UX

Bug fixes:
- Fix config.toml wiped to single provider on switch (now writes all providers)
- Fix managed:kimi-code oauth key corruption (oauth/kimi-code was rewritten
  to oauth/managed-kimi-code, breaking the official subscription)
- Fix default_model not switching due to React 18 deferred state update
  (updateConfig now sets configRef synchronously before setConfig)
- Strip KimiSwitch-private default_model from config.toml provider output

Architecture:
- config.toml is now the authoritative source for Kimi Code provider data
- SQLite stores only metadata (notes, official URLs, remembered models)
- Load reconciles config.toml with SQLite for external /provider changes
- Activate is a no-op for KimiCode (save already writes config.toml)

Features:
- Selected provider moves to top of list on switch
- Add models-dev context size lookup
- Add tauri-plugin-single-instance

Tests:
- 3 new regression tests: oauth key round-trip, all-providers write,
  private default_model strip

src-tauri/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "kimiswitch"
-version = "0.2.0"
+version = "0.3.0"
 description = "Kimi Switch - model config manager"
 authors = ["you"]
 edition = "2021"
@@ -11,6 +11,7 @@ tauri-build = { version = "2.0.0", features = [] }
 [dependencies]
 tauri = { version = "2.0.0", features = ["tray-icon"] }
 tauri-plugin-opener = "2.0.0"
+tauri-plugin-single-instance = "2.0.0"
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 indexmap = { version = "2.2", features = ["serde"] }