revert: restore full config overwrite + Common Config Snippet (revert 992dda5c)

Revert the partial key-field merging refactoring introduced in 992dda5c,
along with two dependent commits (24fa8a18, 87604b18) that referenced
the now-removed ClaudeQuickToggles component.

The whitelist-based partial merge approach had critical issues:
- Non-whitelisted custom fields were lost during provider switching
- Backfill permanently stripped non-key fields from the database
- Whitelist required constant maintenance to track upstream changes

This restores the proven "full config overwrite + Common Config Snippet"
architecture where each provider stores its complete configuration and
shared settings are managed via a separate snippet mechanism.

Reverted commits:
- 24fa8a18: context-aware JSON editor hint + hide quick toggles
- 87604b18: hide ClaudeQuickToggles when creating
- 992dda5c: partial key-field merging refactoring

Restored:
- Full config snapshot write (write_live_snapshot) for Claude/Codex/Gemini
- Full config backfill (settings_config = live_config)
- Common Config Snippet UI and backend commands
- 6 frontend components/hooks for common config editing
- configApi barrel export and DB snippet methods

Removed:
- ClaudeQuickToggles component
- write_live_partial / backfill_key_fields / patch_claude_live
- All KEY_FIELDS constants

src/components/providers/forms/hooks/useApiKeyState.ts

@@ -12,7 +12,6 @@ interface UseApiKeyStateProps {
   selectedPresetId: string | null;
   category?: ProviderCategory;
   appType?: string;
-  apiKeyField?: string;
 }
 
 /**
@@ -25,7 +24,6 @@ export function useApiKeyState({
   selectedPresetId,
   category,
   appType,
-  apiKeyField,
 }: UseApiKeyStateProps) {
   const [apiKey, setApiKey] = useState(() => {
     if (initialConfig) {
@@ -60,7 +58,7 @@ export function useApiKeyState({
         initialConfig || "{}",
         key.trim(),
         {
-          // 最佳实践：仅在"新增模式"且"非官方类别"时补齐缺失字段
+          // 最佳实践：仅在“新增模式”且“非官方类别”时补齐缺失字段
           // - 新增模式：selectedPresetId !== null
           // - 非官方类别：category !== undefined && category !== "official"
           // - 官方类别：不创建字段（UI 也会禁用输入框）
@@ -70,20 +68,12 @@ export function useApiKeyState({
             category !== undefined &&
             category !== "official",
           appType,
-          apiKeyField,
         },
       );
 
       onConfigChange(configString);
     },
-    [
-      initialConfig,
-      selectedPresetId,
-      category,
-      appType,
-      apiKeyField,
-      onConfigChange,
-    ],
+    [initialConfig, selectedPresetId, category, appType, onConfigChange],
   );
 
   const showApiKey = useCallback(