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/index.css

@@ -12,7 +12,7 @@
     --color-text: #e8e6e0;
     --color-text-muted: #878580;
     --color-text-faint: #4a4845;
-    --color-primary: #3b6ed8;
+    --color-primary: #3b82f6;
     --color-pick: #4f9e5c;
     --color-reject: #c25a5a;
     --color-star: #d4a340;