fix(frontend): make infinite scroll viewport-relative, stop eager-loading

Lazy load fetched the entire list at once: every list's loader had a
"fill the viewport" recursion gated on
scrollContainer.scrollHeight <= clientHeight, but <main> is not the
scroller (the window/body is), so that condition is always true and it
recursed through every page (with a 10-item window, ~all pages fired at
once).

Move the filling logic into InfiniteScroll and base it on the sentinel's
viewport rect instead: load while the sentinel is within 300px of the
viewport bottom, re-checked synchronously after each load. This works
regardless of which element scrolls and loads only enough pages to reach
past the viewport.

Drop the per-page recursion (and now-unused scrollContainer refs / tick
imports) from the files, trash, tags, categories and pools lists.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/routes/files/+page.svelte

@@ -203,12 +203,9 @@
 		} finally {
 			loading = false;
 		}
-		// If the loaded content doesn't fill the viewport yet (no scrollbar),
-		// keep loading until it does or there's nothing left.
-		await tick();
-		if (hasMore && scrollContainer && scrollContainer.scrollHeight <= scrollContainer.clientHeight) {
-			void loadMore();
-		}
+		// Viewport filling is handled by InfiniteScroll, which re-checks after each
+		// load — no manual recursion (which over-fetched here because <main> isn't
+		// the scroller, so its scrollHeight never exceeds its clientHeight).
 	}
 
 	function applyFilter(filter: string | null) {