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-tauri/src/database/migration.rs

@@ -58,6 +58,9 @@ impl Database {
         // 4. 迁移 Skills
         Self::migrate_skills(tx, config)?;
 
+        // 5. 迁移 Common Config
+        Self::migrate_common_config(tx, config)?;
+
         Ok(())
     }
 
@@ -209,4 +212,34 @@ impl Database {
 
         Ok(())
     }
+
+    /// 迁移通用配置片段
+    fn migrate_common_config(
+        tx: &rusqlite::Transaction<'_>,
+        config: &MultiAppConfig,
+    ) -> Result<(), AppError> {
+        if let Some(snippet) = &config.common_config_snippets.claude {
+            tx.execute(
+                "INSERT OR REPLACE INTO settings (key, value) VALUES (?1, ?2)",
+                params!["common_config_claude", snippet],
+            )
+            .map_err(|e| AppError::Database(format!("Migrate settings failed: {e}")))?;
+        }
+        if let Some(snippet) = &config.common_config_snippets.codex {
+            tx.execute(
+                "INSERT OR REPLACE INTO settings (key, value) VALUES (?1, ?2)",
+                params!["common_config_codex", snippet],
+            )
+            .map_err(|e| AppError::Database(format!("Migrate settings failed: {e}")))?;
+        }
+        if let Some(snippet) = &config.common_config_snippets.gemini {
+            tx.execute(
+                "INSERT OR REPLACE INTO settings (key, value) VALUES (?1, ?2)",
+                params!["common_config_gemini", snippet],
+            )
+            .map_err(|e| AppError::Database(format!("Migrate settings failed: {e}")))?;
+        }
+
+        Ok(())
+    }
 }