feat(web): use CSS grid-areas layout with anchor-positioned report picker

Replace flexbox layout with CSS grid using named grid-areas for responsive
content containment. Gutters collapse naturally on small screens via
min(--content-max-width, 100%). Anchor ReportPicker to its trigger button
using CSS anchor positioning instead of fixed viewport offsets.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

opal-web/src/lib/components/InputBar.svelte

@@ -119,7 +119,7 @@
 
 <style>
 	.input-bar {
-		flex-shrink: 0;
+		grid-area: input;
 		padding: var(--spacing-sm) var(--spacing-md);
 		padding-bottom: calc(var(--spacing-sm) + env(safe-area-inset-bottom, 0px));
 		background-color: var(--bg-primary);