style(design-system): normalize exact-match border-radius literals to tokens

Mechanical sweep for border-radius: 4px/8px/12px/13px/999px replaced
with var(--radius-xs/sm/md/lg/full, <same value>) across src/app.
Only exact matches to existing token values were touched (20px, 16px,
10px, 6px, 3px, 2px etc. were left as-is since no token maps to them
without a visible size change on at least one tenant theme — see
docs/UI-DESIGN-REVIEW.md).

src/app/components/telegram-login/telegram-login.component.scss

@@ -79,7 +79,7 @@ h2 {
   width: 100%;
   padding: 14px 24px;
   border: none;
-  border-radius: 12px;
+  border-radius: var(--radius-md, 12px);
   background: #2AABEE;
   color: #fff;
   font-size: var(--font-size-lg, 1rem);
@@ -129,12 +129,12 @@ h2 {
     display: inline-flex;
     padding: 12px;
     background: #fff;
-    border-radius: 12px;
+    border-radius: var(--radius-md, 12px);
     border: 1px solid #e8e8e8;
 
     img {
       display: block;
-      border-radius: 4px;
+      border-radius: var(--radius-xs, 4px);
     }
 
     &.qr-loading {