style: improve thumbnail indicator + selection contrast

- Bump primary from #3b6ed8 to #3b82f6 (Tailwind blue-500). Higher
  saturation reads better against both the dark surface and varied
  photo content. Contrast against bg-bg goes from ~5.7:1 to ~6.6:1.
- Selection ring: add ring-offset-2 ring-offset-bg so the bright blue
  has a dark gap separating it from the photo edge — pops on light
  and dark photos alike. Hover ring gets the same treatment.
- Selection check badge: white ring + shadow + thicker stroke so the
  badge is legible against any photo (was disappearing on bright
  scenes).
- Rating stars: wrap in a translucent dark pill with backdrop-blur so
  yellow stars don't vanish on yellow / sandy photos.
- Heap / duplicate / discarded badges: matching white ring + thicker
  icon stroke so they all read consistently and don't blend in.
- Timeline date headers: text-text instead of text-text-muted so the
  group labels actually pop above the grid.

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

frontend/src/components/timeline/Timeline.tsx

@@ -482,7 +482,7 @@ export function Timeline() {
                   }}
                   className="flex items-end pb-1"
                 >
-                  <h3 className="text-sm font-semibold uppercase tracking-wide text-text-muted">
+                  <h3 className="text-sm font-semibold uppercase tracking-wide text-text">
                     {item.label}
                   </h3>
                 </div>