refactor(i18n): complete error message internationalization and code cleanup

Replaced all remaining hardcoded error types with AppError::localized for
full bilingual support and simplified error handling logic.

Backend changes:
- usage_script.rs: Converted InvalidHttpMethod to localized error
- provider.rs: Replaced all 7 ProviderNotFound instances with localized errors
  * Line 436: Delete provider validation
  * Line 625: Update provider metadata
  * Line 785: Test usage script provider lookup
  * Line 855: Query usage provider lookup
  * Line 924: Prepare Codex provider switch
  * Line 1011: Prepare Claude provider switch
  * Line 1272: Delete provider snapshot
- provider.rs: Simplified error message formatting (removed 40+ lines)
  * Removed redundant string matching fallback logic
  * Now uses clean language-based selection for Localized errors
  * Falls back to default Display for other error types
- error.rs: Removed unused error variants
  * Deleted InvalidHttpMethod (replaced with localized)
  * Deleted ProviderNotFound (replaced with localized)

Code quality improvements:
- Reduced complexity: 40+ lines of string matching removed
- Better maintainability: Centralized error message handling
- Type safety: All provider errors now use consistent localized format

Impact:
- 100% i18n coverage for provider and usage script error messages
- Cleaner, more maintainable error handling code
- No unused error variants remaining

src-tauri/src/error.rs

@@ -40,14 +40,10 @@ pub enum AppError {
     },
     #[error("锁获取失败: {0}")]
     Lock(String),
-    #[error("供应商不存在: {0}")]
-    ProviderNotFound(String),
     #[error("MCP 校验失败: {0}")]
     McpValidation(String),
     #[error("{0}")]
     Message(String),
-    #[error("不支持的 HTTP 方法: {0}")]
-    InvalidHttpMethod(String),
     #[error("{zh} ({en})")]
     Localized {
         key: &'static str,