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/tailwind.config.js

@@ -17,7 +17,7 @@ export default {
         text: '#e8e6e0',
         'text-muted': '#878580',
         'text-faint': '#4a4845',
-        primary: '#3b6ed8', // deep royal blue
+        primary: '#3b82f6', // saturated blue (Tailwind blue-500) — high contrast on dark bg AND on photo content
         pick: '#4f9e5c',    // green for picked
         reject: '#c25a5a',  // red for rejected
         star: '#d4a340',    // amber for stars