feat: custom theme system with PostgreSQL PL/pgSQL highlighting fix

- Fix PostgreSQL function highlighting by disabling doubleDollarQuotedStrings
  in extended PostgreSQL dialect, enabling PL/pgSQL syntax highlighting
  inside $ blocks
- Add complete custom editor theme system with multi-theme management
  (create, rename, duplicate, delete), visual color editor (12 colors),
  JSON import/export, real-time preview, and 12 preset color schemes
- Add background/foreground color customization with system theme defaults
- Optimize EditorSettingsDialog layout: 2-column grid with font selector
  taking available space and theme dropdown grouped with custom theme button
- Add i18n support for custom theme UI (en, zh-CN, zh-TW, es)
- Fix Tauri production build by adding custom-protocol feature
- Update .gitignore for temporary build artifacts

Closes t8y2#788

Co-authored-by: Sam <14344444@@qq.com>

.cargo/config.toml

@@ -1,5 +1,4 @@
 [env]
-# Enable SQLite's built-in math functions for rusqlite's bundled libsqlite3.
 LIBSQLITE3_FLAGS = "SQLITE_ENABLE_MATH_FUNCTIONS"
 
 [target.x86_64-pc-windows-msvc]