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/tests.rs

@@ -513,6 +513,8 @@ fn schema_dry_run_does_not_write_to_disk() {
         mcp: Default::default(),
         prompts: Default::default(),
         skills: Default::default(),
+        common_config_snippets: Default::default(),
+        claude_common_config_snippet: None,
     };
 
     // Dry-run should succeed without any file I/O errors
@@ -561,6 +563,8 @@ fn dry_run_validates_schema_compatibility() {
         mcp: Default::default(),
         prompts: Default::default(),
         skills: Default::default(),
+        common_config_snippets: Default::default(),
+        claude_common_config_snippet: None,
     };
 
     // Dry-run should validate the full migration path