feat: add anthropic-compatible provider type

Add a new provider type "anthropic-compatible" to support Anthropic API
variants that use array format for the system field instead of string.

Changes:
- Add AnthropicCompatible to ApiProviderType enum (backend)
- Add AnthropicCompatible to Agent ProviderType enum
- Modify AnthropicProtocol to support array system format
- Add provider type selector in ProviderConfigForm for custom providers
- Update icon mappings to show Claude icon for anthropic-compatible
- Update model registry mappings to show Anthropic models

This allows users to connect to Anthropic-compatible proxies (like One API)
that expect the system field in array format:
  {"system": [{"type": "text", "text": "..."}]}

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src-tauri/tests/api_key_provider_tests.rs

@@ -703,14 +703,15 @@ mod unit_tests {
                 &ctx.db,
                 &provider.id,
                 Some("Updated Name".to_string()),
-                None,
-                Some(false),
-                None,
-                None,
-                None,
-                None,
-                None,
-                None,
+                None,        // provider_type
+                None,        // api_host
+                Some(false), // enabled
+                None,        // sort_order
+                None,        // api_version
+                None,        // project
+                None,        // location
+                None,        // region
+                None,        // custom_models
             )
             .expect("Failed to update provider");