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)

assets/oh-my-opencode.schema.json

@@ -57,7 +57,9 @@
           "startup-toast",
           "keyword-detector",
           "agent-usage-reminder",
-          "non-interactive-env"
+          "non-interactive-env",
+          "interactive-bash-session",
+          "empty-message-sanitizer"
         ]
       }
     },