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/usage_script.rs

@@ -137,7 +137,13 @@ async fn send_http_request(config: &RequestConfig, timeout_secs: u64) -> Result<
     let method: reqwest::Method = config
         .method
         .parse()
-        .map_err(|_| AppError::InvalidHttpMethod(config.method.clone()))?;
+        .map_err(|_| {
+            AppError::localized(
+                "usage_script.invalid_http_method",
+                format!("不支持的 HTTP 方法: {}", config.method),
+                format!("Unsupported HTTP method: {}", config.method),
+            )
+        })?;
 
     let mut req = client.request(method.clone(), &config.url);