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/GeminiCommonConfigModal.tsx

@@ -0,0 +1,106 @@
+import React, { useEffect, useState } from "react";
+import { Save, Download, Loader2 } from "lucide-react";
+import { useTranslation } from "react-i18next";
+import { FullScreenPanel } from "@/components/common/FullScreenPanel";
+import { Button } from "@/components/ui/button";
+import JsonEditor from "@/components/JsonEditor";
+
+interface GeminiCommonConfigModalProps {
+  isOpen: boolean;
+  onClose: () => void;
+  value: string;
+  onChange: (value: string) => void;
+  error?: string;
+  onExtract?: () => void;
+  isExtracting?: boolean;
+}
+
+/**
+ * GeminiCommonConfigModal - Common Gemini configuration editor modal
+ * Allows editing of common env snippet shared across Gemini providers
+ */
+export const GeminiCommonConfigModal: React.FC<
+  GeminiCommonConfigModalProps
+> = ({ isOpen, onClose, value, onChange, error, onExtract, isExtracting }) => {
+  const { t } = useTranslation();
+  const [isDarkMode, setIsDarkMode] = useState(false);
+
+  useEffect(() => {
+    setIsDarkMode(document.documentElement.classList.contains("dark"));
+
+    const observer = new MutationObserver(() => {
+      setIsDarkMode(document.documentElement.classList.contains("dark"));
+    });
+
+    observer.observe(document.documentElement, {
+      attributes: true,
+      attributeFilter: ["class"],
+    });
+
+    return () => observer.disconnect();
+  }, []);
+
+  return (
+    <FullScreenPanel
+      isOpen={isOpen}
+      title={t("geminiConfig.editCommonConfigTitle", {
+        defaultValue: "编辑 Gemini 通用配置片段",
+      })}
+      onClose={onClose}
+      footer={
+        <>
+          {onExtract && (
+            <Button
+              type="button"
+              variant="outline"
+              onClick={onExtract}
+              disabled={isExtracting}
+              className="gap-2"
+            >
+              {isExtracting ? (
+                <Loader2 className="w-4 h-4 animate-spin" />
+              ) : (
+                <Download className="w-4 h-4" />
+              )}
+              {t("geminiConfig.extractFromCurrent", {
+                defaultValue: "从编辑内容提取",
+              })}
+            </Button>
+          )}
+          <Button type="button" variant="outline" onClick={onClose}>
+            {t("common.cancel")}
+          </Button>
+          <Button type="button" onClick={onClose} className="gap-2">
+            <Save className="w-4 h-4" />
+            {t("common.save")}
+          </Button>
+        </>
+      }
+    >
+      <div className="space-y-4">
+        <p className="text-sm text-muted-foreground">
+          {t("geminiConfig.commonConfigHint", {
+            defaultValue:
+              "该片段会写入 Gemini 的 .env（不允许包含 GOOGLE_GEMINI_BASE_URL、GEMINI_API_KEY）",
+          })}
+        </p>
+
+        <JsonEditor
+          value={value}
+          onChange={onChange}
+          placeholder={`{
+  "GEMINI_MODEL": "gemini-3-pro-preview"
+}`}
+          darkMode={isDarkMode}
+          rows={16}
+          showValidation={true}
+          language="json"
+        />
+
+        {error && (
+          <p className="text-sm text-red-500 dark:text-red-400">{error}</p>
+        )}
+      </div>
+    </FullScreenPanel>
+  );
+};