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)

packages/design-system/src/css/style.css

@@ -634,6 +634,10 @@
   }
 }
 
+@utility bg-subscription-gradient {
+  background: var(--color-subscription-button-gradient);
+}
+
 @utility highlight {
   background-color: color-mix(in srgb, currentColor 20%, transparent);
   font-weight: 700;