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/SearchModal.vue

@@ -146,7 +146,7 @@ onUnmounted(() => document.removeEventListener('keydown', onKeydown));
   align-self: flex-start;
   background: var(--aurora-frame-bg, #1c1c20);
   border: 1px solid var(--aurora-frame-border, #34343b);
-  border-radius: 14px;
+  border-radius: var(--aurora-radius-md);
   box-shadow: 0 24px 64px rgba(0, 0, 0, 0.45);
   overflow: hidden;
   font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
@@ -171,7 +171,7 @@ onUnmounted(() => document.removeEventListener('keydown', onKeydown));
   font-size: 0.6875rem;
   color: var(--aurora-text-muted);
   border: 1px solid var(--aurora-border, #27272a);
-  border-radius: 5px;
+  border-radius: var(--aurora-radius-sm);
   padding: 2px 6px;
 }
 .smodal__state {
@@ -197,7 +197,7 @@ onUnmounted(() => document.removeEventListener('keydown', onKeydown));
   cursor: pointer;
   text-align: left;
   padding: 10px 12px;
-  border-radius: 9px;
+  border-radius: var(--aurora-radius-md);
 }
 .smodal__result--active { background: var(--aurora-accent-soft); }
 .smodal__result-title {