fix(webui): ARIA-valid hover text, alias metadata guard, context-chip polish

CompactNumber.vue and ProviderRateBadge.vue both put aria-label on a
plain <span> — axe's aria-prohibited-attr rule: a <span>'s implicit
ARIA role ("generic") does not support accessible-name computation at
all, so the attribute was simply invalid there, not just discouraged.
Both now use the visible-text-hidden-plus-sr-only-full-text pattern
instead: the compact/label text is marked aria-hidden="true", and a
sr-only sibling span carries the exact value/detail for assistive tech.
title stays on the outer element for mouse hover, unchanged. Folded
review minor.

ProvidersView.vue: the alias table's ModelChip cell read
row.original.modelMeta.contextTokens without a guard, unlike the
provider path's own modelMetaFor fallback — a response from a server
build older than this field (version skew) would throw instead of just
omitting the hover detail. Now optional-chained to match. The visible
context chip beside each model gains title="context window" and
text-xs (was an arbitrary text-[10px]), matching the rest of the panel's
muted-label sizing convention. Folded review minors.

webui/src/components/CompactNumber.vue

@@ -5,13 +5,22 @@ import { formatCompactCount, formatExactInt } from '@/lib/format'
 
 /**
  * CompactNumber renders a large raw token/request count in its compact
- * SI-style form ("1.23M") while carrying the exact value via both
- * `title` (mouse hover) and `aria-label` (keyboard/screen-reader parity
- * — the same established pattern ProviderRateBadge.vue and ModelChip.vue
- * already apply to their own hover detail). Used anywhere a usage count
- * renders: DataTable numeric columns (usage-columns.ts, target-
- * columns.ts, via `h(CompactNumber, { value })`), the Usage tab's Total
- * card, and a group accordion's trigger/detail stat grid (UsageView.vue).
+ * SI-style form ("1.23M") while carrying the exact value via `title`
+ * (mouse hover) and a visually-hidden sibling span (keyboard/screen-
+ * reader parity). Used anywhere a usage count renders: DataTable numeric
+ * columns (usage-columns.ts, target-columns.ts, via
+ * `h(CompactNumber, { value })`), the Usage tab's Total card, and a
+ * group accordion's trigger/detail stat grid (UsageView.vue).
+ *
+ * Review fix (folded minor, ARIA): `aria-label` on a plain `<span>` is
+ * an axe `aria-prohibited-attr` violation — a `<span>`'s implicit ARIA
+ * role ("generic") does not support name computation at all, so
+ * `aria-label`/`aria-labelledby` there is simply invalid, not merely
+ * discouraged. The visible, compact text is marked `aria-hidden="true"`
+ * so assistive tech skips the abbreviated form, and a `sr-only` sibling
+ * span (visually hidden, still in the accessibility tree) carries the
+ * exact value instead — the same visible-abbreviation-plus-hidden-full-
+ * text pattern ProviderRateBadge.vue now applies too.
  *
  * Cost figures are NOT this component's concern — formatCost already
  * shows full 4-decimal precision, so there is nothing to compact or
@@ -24,5 +33,8 @@ const exact = computed(() => formatExactInt(props.value))
 </script>
 
 <template>
-  <span :title="exact" :aria-label="exact">{{ compact }}</span>
+  <span :title="exact">
+    <span aria-hidden="true">{{ compact }}</span>
+    <span class="sr-only">{{ exact }}</span>
+  </span>
 </template>