fix: make credits help icon a tooltip button in cloud user popover

Wrap the bare <i class="icon-[lucide--circle-help]"> next to the
credits balance in a <Button variant="muted-textonly" size="icon-sm">
so PrimeVue's v-tooltip fires reliably and the icon gets proper
hover/focus states and keyboard accessibility. Apply the same change to
CurrentUserPopoverLegacy.vue and CurrentUserPopoverWorkspace.vue, which
shared the same broken pattern.

Fixes FE-617

src/components/topbar/CurrentUserPopoverLegacy.vue

@@ -41,10 +41,16 @@
       <span v-else class="text-base font-semibold text-base-foreground">{{
         formattedBalance
       }}</span>
-      <i
+      <Button
         v-tooltip="{ value: $t('credits.unified.tooltip'), showDelay: 300 }"
-        class="mr-auto icon-[lucide--circle-help] cursor-help text-base text-muted-foreground"
-      />
+        variant="muted-textonly"
+        size="icon-sm"
+        class="mr-auto"
+        :aria-label="$t('credits.unified.tooltip')"
+        data-testid="credits-info-button"
+      >
+        <i class="icon-[lucide--circle-help]" />
+      </Button>
       <Button
         v-if="isCloud && isFreeTier"
         variant="gradient"

src/platform/workspace/components/CurrentUserPopoverWorkspace.vue

@@ -68,10 +68,16 @@
       <span v-else class="text-base font-semibold text-base-foreground">{{
         displayedCredits
       }}</span>
-      <i
+      <Button
         v-tooltip="{ value: $t('credits.unified.tooltip'), showDelay: 300 }"
-        class="mr-auto icon-[lucide--circle-help] cursor-help text-base text-muted-foreground"
-      />
+        variant="muted-textonly"
+        size="icon-sm"
+        class="mr-auto"
+        :aria-label="$t('credits.unified.tooltip')"
+        data-testid="credits-info-button"
+      >
+        <i class="icon-[lucide--circle-help]" />
+      </Button>
       <!-- Upgrade to add credits (free tier) -->
       <Button
         v-if="isActiveSubscription && permissions.canTopUp && isFreeTier"