Add GUI list/gallery wallpaper browsing modes

[Copilot]

The GUI wallpaper browser was locked to a split list/preview layout. This change adds a gallery mode alongside the existing list mode, with top-level All / Rotation tabs in gallery view and [ / ] keyboard navigation between those tabs.

• View mode toggle

  • Added a GUI List / Gallery toggle in the toolbar.
  • Preserved the current split-pane browser as List view.

• Gallery layout

  • Added a thumbnail gallery in the main content area.
  • Removed the separate right-side preview in gallery mode so wallpapers are shown directly in the grid.
  • Kept selection and double-click apply behavior consistent with the existing browser.

• Section navigation

  • Refactored section tab rendering so All and Rotation can be reused across views.
  • Added [ and ] shortcuts to cycle the active gallery tab with wraparound behavior.

• Preview/thumb handling

  • Reused the existing preview pipeline to load gallery thumbnails via the GUI list thumbnail profile.
  • Increased GUI texture cache capacity to better support gallery browsing.

• Docs and help

  • Updated GUI help text and README to describe the new browsing modes and bracket-key tab navigation.

Example of the new keyboard behavior:

if ctx.input_mut(|input| input.consume_key(Modifiers::NONE, Key::CloseBracket)) {
    self.cycle_active_section(1);
}
if ctx.input_mut(|input| input.consume_key(Modifiers::NONE, Key::OpenBracket)) {
    self.cycle_active_section(-1);
}

Original prompt

Add the ability to switching how we view wallpapers. currently, there is a side panel that lists the wallpaper and the right side that shows the wallpaper itself. I want to switch to a gallery view and have the ability to toggle between the two. gallery view will have tabs at the top(nav by [ or ]), to switch between full list and rotation list. there will be no preview(right side in list view) since wallpapers are all being shown in gallery view.