feat: desert dusk theme with animated mule and pixel-art scenery

Replaces the cool-grey neutral theme with a warm desert dusk palette
pulled from a new pixel-art TopBar: a tiled, right-to-left scrolling
desert under a sky gradient, a 6-frame walking mule sprite where the
logo used to sit, and an ASCII block-character title on a black plate.
The active heap card now uses a cactus/dune scene as its stack
backdrop, and the "Built with hubris" mark moves into the TopBar's
bottom-right corner (AppFooter component removed).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/App.tsx

@@ -7,7 +7,6 @@ import { TopBar } from './components/layout/TopBar'
 import { ScanProgress } from './components/ScanProgress'
 import { ToastContainer } from './components/ToastContainer'
 import { KeyboardHints } from './components/KeyboardHints'
-import { AppFooter } from './components/AppFooter'
 import { PreviewView } from './components/preview/PreviewView'
 import { FilterBar } from './components/filter/FilterBar'
 import { DiscardActionBar } from './components/discard/DiscardActionBar'
@@ -84,7 +83,6 @@ function App() {
            *  glassy. Mounted here so it's centered against the timeline,
            *  not the viewport (which would be offset by the sidebars). */}
           <KeyboardHints />
-          <AppFooter />
         </div>
 
         {/* Right Sidebar */}