fix: add Windows config path documentation and config error warnings (#97) (#109)

- Document platform-specific config paths in README (en/ko/ja)
  - Windows: %APPDATA%\opencode\oh-my-opencode.json
  - macOS/Linux: ~/.config/opencode/oh-my-opencode.json
- Show config file path in startup toast
- Add config load error warnings when JSON parsing or validation fails
- Extract getUserConfigDir to shared/config-path.ts for reuse

Fixes #97

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/shared/index.ts

@@ -10,3 +10,4 @@ export * from "./hook-disabled"
 export * from "./deep-merge"
 export * from "./file-utils"
 export * from "./dynamic-truncator"
+export * from "./config-path"