style: switch primary accent from teal to deep royal blue

Replace #4f98a3 (desaturated teal) with #3b6ed8 (deep royal blue) as
the app's primary accent. Affects every text-primary, bg-primary,
ring-primary, border-primary class — selection rings, active sidebar
rows, the active filter pill background, the loupe info button when
open, etc.

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: #4f98a3;
+    --color-primary: #3b6ed8;
     --color-pick: #4f9e5c;
     --color-reject: #c25a5a;
     --color-star: #d4a340;

frontend/tailwind.config.js

@@ -17,7 +17,7 @@ export default {
         text: '#e8e6e0',
         'text-muted': '#878580',
         'text-faint': '#4a4845',
-        primary: '#4f98a3', // desaturated teal
+        primary: '#3b6ed8', // deep royal blue
         pick: '#4f9e5c',    // green for picked
         reject: '#c25a5a',  // red for rejected
         star: '#d4a340',    // amber for stars