TP-18 — Add `ThemePickerButton` widget shell

[ZhuchkaTriplesix]

Epic: C
Depends on: TP-10

Source: docs/theme-parser-github-issues.md

Goal

Introduce a dedicated picker UI for many themes instead of overloading a small dropdown.

Files

• lib/features/settings/theme_picker_button.dart
• test/features/settings/theme_picker_button_test.dart

Implementation

Create widget:

class ThemePickerButton extends StatelessWidget {
  const ThemePickerButton({
    required this.themes,
    required this.selectedThemeId,
    required this.onSelected,
    this.isLoading = false,
  });
}

Use:

• MenuAnchor;
• fixed/max popup height 300-360px;
• Scrollbar;
• ListView.builder;
• row shows:
  • name;
  • source badge;
  • dark/light icon or label.

Acceptance Criteria

• Opens menu with 50+ fake themes without overflow.
• Uses builder list, not Column(children: themes.map(...)).
• Does not parse or apply theme during build.

Tests

• pump with 60 definitions;
• open menu;
• visible rows render;
• no exception/overflow in test logs if test harness supports it;
• tap row triggers onSelected(id).

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