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/GeminiConfigEditor.tsx

@@ -1,5 +1,6 @@
-import React from "react";
+import React, { useState, useEffect } from "react";
 import { GeminiEnvSection, GeminiConfigSection } from "./GeminiConfigSections";
+import { GeminiCommonConfigModal } from "./GeminiCommonConfigModal";
 
 interface GeminiConfigEditorProps {
   envValue: string;
@@ -7,8 +8,15 @@ interface GeminiConfigEditorProps {
   onEnvChange: (value: string) => void;
   onConfigChange: (value: string) => void;
   onEnvBlur?: () => void;
+  useCommonConfig: boolean;
+  onCommonConfigToggle: (checked: boolean) => void;
+  commonConfigSnippet: string;
+  onCommonConfigSnippetChange: (value: string) => void;
+  commonConfigError: string;
   envError: string;
   configError: string;
+  onExtract?: () => void;
+  isExtracting?: boolean;
 }
 
 const GeminiConfigEditor: React.FC<GeminiConfigEditorProps> = ({
@@ -17,9 +25,25 @@ const GeminiConfigEditor: React.FC<GeminiConfigEditorProps> = ({
   onEnvChange,
   onConfigChange,
   onEnvBlur,
+  useCommonConfig,
+  onCommonConfigToggle,
+  commonConfigSnippet,
+  onCommonConfigSnippetChange,
+  commonConfigError,
   envError,
   configError,
+  onExtract,
+  isExtracting,
 }) => {
+  const [isCommonConfigModalOpen, setIsCommonConfigModalOpen] = useState(false);
+
+  // Auto-open common config modal if there's an error
+  useEffect(() => {
+    if (commonConfigError && !isCommonConfigModalOpen) {
+      setIsCommonConfigModalOpen(true);
+    }
+  }, [commonConfigError, isCommonConfigModalOpen]);
+
   return (
     <div className="space-y-6">
       {/* Env Section */}
@@ -28,6 +52,10 @@ const GeminiConfigEditor: React.FC<GeminiConfigEditorProps> = ({
         onChange={onEnvChange}
         onBlur={onEnvBlur}
         error={envError}
+        useCommonConfig={useCommonConfig}
+        onCommonConfigToggle={onCommonConfigToggle}
+        onEditCommonConfig={() => setIsCommonConfigModalOpen(true)}
+        commonConfigError={commonConfigError}
       />
 
       {/* Config JSON Section */}
@@ -36,6 +64,17 @@ const GeminiConfigEditor: React.FC<GeminiConfigEditorProps> = ({
         onChange={onConfigChange}
         configError={configError}
       />
+
+      {/* Common Config Modal */}
+      <GeminiCommonConfigModal
+        isOpen={isCommonConfigModalOpen}
+        onClose={() => setIsCommonConfigModalOpen(false)}
+        value={commonConfigSnippet}
+        onChange={onCommonConfigSnippetChange}
+        error={commonConfigError}
+        onExtract={onExtract}
+        isExtracting={isExtracting}
+      />
     </div>
   );
 };