Round corners a notch; tokenize all radii onto an sm/md/lg/xl scale

Consolidate ~140 scattered hardcoded border-radius values (and the old
--aurora-radius/--aurora-radius-lg vars) onto one four-step scale defined in
shell-frame.css:
  --aurora-radius-sm 10px  (chips, small/icon controls; was 5-8px)
  --aurora-radius-md 14px  (buttons, inputs, search, list rows; was 9-14px)
  --aurora-radius-lg 20px  (cards, panels, larger buttons; was 15-20px)
  --aurora-radius-xl 28px  (page frames: content card, dashboard, sidebar,
                            editor; was the 24px --aurora-radius-lg)
Everything is one notch rounder, and the whole UI's roundness is now tunable
from four lines instead of 35 files.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

app/src/shell/components/Toolbar.vue

@@ -102,7 +102,7 @@ onUnmounted(() => document.removeEventListener('keydown', onKeydown));
   justify-content: center;
   width: 34px;
   height: 34px;
-  border-radius: 8px;
+  border-radius: var(--aurora-radius-sm);
   color: var(--aurora-text-muted);
   text-decoration: none;
 }
@@ -120,7 +120,7 @@ onUnmounted(() => document.removeEventListener('keydown', onKeydown));
   padding: 0 10px;
   min-width: 240px;
   border: 1px solid var(--aurora-border);
-  border-radius: 10px;
+  border-radius: var(--aurora-radius-md);
   background: var(--aurora-bg-subtle);
   color: var(--aurora-text-muted);
   font-size: 0.8125rem;
@@ -141,7 +141,7 @@ onUnmounted(() => document.removeEventListener('keydown', onKeydown));
 .toolbar__search-kbd {
   font-size: 0.6875rem;
   border: 1px solid var(--aurora-border);
-  border-radius: 5px;
+  border-radius: var(--aurora-radius-sm);
   padding: 1px 6px;
   background: var(--aurora-bg);
 }
@@ -154,7 +154,7 @@ onUnmounted(() => document.removeEventListener('keydown', onKeydown));
   height: 34px;
   border: none;
   background: none;
-  border-radius: 8px;
+  border-radius: var(--aurora-radius-sm);
   cursor: pointer;
   color: var(--aurora-text-muted);
 }