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;