feat: add ever-present upgrade button for free-tier users (#9315)

## Summary

Add persistent upgrade CTAs for free-tier users: a topbar button and
"Upgrade to add credits" replacing "Add Credits" in popovers and
settings panels.

## Changes

- **What**:
- New `TopbarSubscribeButton` component in both GraphCanvas and
LinearView topbars, visible only to free-tier users
- Profile popover (legacy + workspace): free-tier users see "Upgrade to
add credits" instead of "Add Credits", linking directly to the pricing
table
- Manage Plan settings (legacy + workspace): same replacement —
free-tier users see "Upgrade to add credits" instead of "Add Credits"
- Paid-tier users retain the original "Add Credits" behavior in all
locations
  - All upgrade buttons go directly to the pricing table (one-step flow)

## Review Focus

- The `isFreeTier` conditional gating on the buttons — ensure free-tier
users see upgrade CTAs and paid users see normal Add Credits
- Layout in Manage Plan panels uses `flex flex-col gap-3` to stack the
upgrade button below the usage history link instead of side-by-side

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9315-feat-add-ever-present-upgrade-button-for-free-tier-users-3166d73d365081228cdfe6a67fec6aec)
by [Unito](https://www.unito.io)

src/platform/workspace/components/SubscriptionPanelContentWorkspace.vue

@@ -252,9 +252,18 @@
                     !showZeroState &&
                     permissions.canTopUp
                   "
-                  class="flex items-center justify-between"
+                  class="flex flex-col gap-3"
                 >
                   <Button
+                    v-if="isFreeTierPlan"
+                    variant="gradient"
+                    class="p-2 min-h-8 rounded-lg text-sm font-normal w-full"
+                    @click="handleUpgradeToAddCredits"
+                  >
+                    {{ $t('subscription.upgradeToAddCredits') }}
+                  </Button>
+                  <Button
+                    v-else
                     variant="secondary"
                     class="p-2 min-h-8 rounded-lg text-sm font-normal text-text-primary bg-interface-menu-component-surface-selected"
                     @click="handleAddApiCredits"
@@ -464,6 +473,10 @@ function handleSubscribeWorkspace() {
 function handleUpgrade() {
   isFreeTierPlan.value ? showPricingTable() : showSubscriptionDialog()
 }
+
+function handleUpgradeToAddCredits() {
+  showPricingTable()
+}
 const subscriptionTier = computed(() => subscription.value?.tier ?? null)
 const isYearlySubscription = computed(
   () => subscription.value?.duration === 'ANNUAL'