refactor: migrate all Tauri commands to camelCase parameters

This commit addresses parameter naming inconsistencies caused by Tauri v2's
requirement for camelCase parameter names in IPC commands.

Backend changes (Rust):
- Updated all command parameters from snake_case to camelCase
- Commands affected:
  * provider.rs: providerId (×4), timeoutSecs
  * import_export.rs: filePath (×2), defaultName
  * config.rs: defaultPath
- Added #[allow(non_snake_case)] attributes for camelCase parameters
- Removed unused QueryUsageParams struct

Frontend changes (TypeScript):
- Removed redundant snake_case parameters from all invoke() calls
- Updated API files:
  * usage.ts: removed debug logs, unified to providerId
  * vscode.ts: updated 8 functions (providerId, timeoutSecs, filePath, defaultName)
  * settings.ts: updated 4 functions (defaultPath, filePath, defaultName)
- Ensured all parameters now use camelCase exclusively

Test updates:
- Updated MSW handlers to accept both old and new parameter formats during transition
- Added i18n mock compatibility for tests

Root cause:
The issue stemmed from Tauri v2 strictly requiring camelCase for command
parameters, while the codebase was using snake_case. This caused parameters
like 'provider_id' to not be recognized by the backend, resulting in
"missing providerId parameter" errors.

BREAKING CHANGE: All Tauri command invocations now require camelCase parameters.
Any external tools or scripts calling these commands must be updated accordingly.

Fixes: Usage query always failing with "missing providerId" error
Fixes: Custom endpoint management not receiving provider ID
Fixes: Import/export dialogs not respecting default paths

src/lib/api/usage.ts

@@ -7,7 +7,7 @@ export const usageApi = {
   async query(providerId: string, appId: AppId): Promise<UsageResult> {
     try {
       return await invoke("query_provider_usage", {
-        provider_id: providerId,
+        providerId: providerId,
         app: appId,
       });
     } catch (error: unknown) {