Improve User Rules UI/UX with RulesEditor

### Issue Details

User rules management lacks basic tooling (search, quick enable/disable),

### Proposed solution

1. View mode and Edit mode:
1.1 **View mode (default)**: powered by `RulesEditor` with search and filtering. Read-only.
1.2 **Edit mode**: `Ace` editor for manual input, same as today.

Switching between modes preserves input.

2. Disabled state indication. When the `User rules` toggle is off, show a clear visual indication (banner, muted styling) that rules are not active.

3. Hamburger menu for secondary actions. Move import, export, delete all rules, and syntax docs link out of the main toolbar into a secondary menu.

4. Empty and transitional states:
4.1 **No rules**: empty state with a prompt to add or import rules
4.2 **Saving**: loading indicator
4.3 **Error**: inline error with retry action

5. Fullscreen mode. Update fullscreen layout to match the new design: mode switcher, hamburger menu, state screens.

6. Snackbars. Add toast notifications for: save, import, export, delete all, and save failure.

### Alternative solution

Keep `Ace` with minor improvements (hints, docs links, basic validation). Less disruptive, but does not solve the core problem and keeps the extension inconsistent with other AdGuard products.