fix(styles): use theme tokens for hardcoded brand blues (#345)

Replace hardcoded #1d4ed8 / #2563eb / #bfdbfe values with their exact
design-token equivalents (--primary, --primary-hover, --primary-strong)
in 17 declarations across auth, dark, management, and vault styles.

Light theme is pixel-identical: every replaced hex equals the token's
light value. In dark theme this fixes spots that dark.css never
overrode and that kept light-theme blues on dark backgrounds:
standalone footer links and version badge, JWT warning inline link,
restore-progress active dot, TOTP countdown ring, and the
authorized-device checkbox accent.

Intentionally left alone: .btn-primary gradients (would lighten dark
buttons under white text), card brand colors (Amex/Maestro/RuPay blues
are brand constants, not theme colors), and light-pill pairings whose
backgrounds have no token (.log-mode-option.active, .log-category-auth,
.log-level-info, .folder-edit-btn:hover, #93c5fd borders).

webapp/src/styles/dark.css

@@ -220,7 +220,7 @@
 }
 
 :root[data-theme='dark'] .card-brand-icon {
-  color: #bfdbfe;
+  color: var(--primary-strong);
   background: linear-gradient(180deg, #1f2937 0%, #111827 100%);
   border-color: color-mix(in srgb, var(--primary) 30%, var(--line));
 }