feat: virtual scrolling for gallery grid view

[msedek]

Summary

Implements row-based virtual scrolling for the gallery card grid using @tanstack/vue-virtual, addressing the performance issue described in #1426 and #3161.

Problem

When browsing a platform with 1000+ ROMs, the infinite scroll keeps appending GameCard components to the DOM. After scrolling 500+ cards (~25,000 DOM nodes), clicking a game freezes the browser as Vue destroys all components at once.

Solution

• Replace the flat v-for over filteredRoms with useWindowVirtualizer from @tanstack/vue-virtual
• ROMs are grouped into rows based on responsive column count (derived from Vuetify breakpoints + view settings)
• Only visible rows + 5 overscan rows are rendered in DOM
• At 1000 ROMs with 6 columns: ~60 cards in DOM instead of 1000

What changed

• frontend/src/views/Gallery/Platform.vue — template and script
• frontend/package.json — added @tanstack/vue-virtual dependency

What didn't change

• ROM store, GameCard component, infinite scroll fetch logic, selection logic — all untouched
• List/table view (currentView == 2) — unchanged

Test plan

• Browse a platform with 1000+ ROMs — should render smoothly
• Scroll through all ROMs — only visible cards in DOM (inspect Elements tab)
• Click a game after scrolling 500+ cards — instant navigation, no freeze
• Responsive: resize window — columns adjust correctly
• View switching (small/big/list) — all views work
• Multi-select with shift-click — selection logic preserved

Tested with 5,600 ROMs across 12 platforms (largest: NES with 1,366 ROMs).

AI-assisted: Claude was used for research and implementation.

Closes #3161

[zurdi15]

Can you fix the Typescript check

Also, is it possible for you to sign the commit? It's OK otherwise