Improve run creation workflow with filters, local box art, and sticky nav

Use local /boxart/{slug}.png images instead of database boxArtUrl with
color-swatch fallback. Add region filter pills and run-status checkboxes
(hide active/completed) to GameGrid. Move the Next button into a sticky
top bar showing selected game summary so it's always visible. Capitalize
region names in all display locations.

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

frontend/src/pages/RunEncounters.tsx

@@ -580,7 +580,7 @@ export function RunEncounters() {
               {run.name}
             </h1>
             <p className="text-gray-600 dark:text-gray-400 mt-1">
-              {run.game.name} &middot; {run.game.region} &middot; Started{' '}
+              {run.game.name} &middot; {run.game.region.charAt(0).toUpperCase() + run.game.region.slice(1)} &middot; Started{' '}
               {new Date(run.startedAt).toLocaleDateString(undefined, {
                 year: 'numeric',
                 month: 'short',