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/components/ui/button/button.variants.ts

@@ -19,7 +19,9 @@ export const buttonVariants = cva({
         'text-muted-foreground bg-transparent hover:bg-secondary-background-hover',
       'destructive-textonly':
         'text-destructive-background bg-transparent hover:bg-destructive-background/10',
-      'overlay-white': 'bg-white text-gray-600 hover:bg-white/90'
+      'overlay-white': 'bg-white text-gray-600 hover:bg-white/90',
+      gradient:
+        'bg-subscription-gradient text-white border-transparent hover:opacity-90'
     },
     size: {
       sm: 'h-6 rounded-sm px-2 py-1 text-xs',
@@ -47,7 +49,8 @@ const variants = [
   'textonly',
   'muted-textonly',
   'destructive-textonly',
-  'overlay-white'
+  'overlay-white',
+  'gradient'
 ] as const satisfies Array<ButtonVariants['variant']>
 const sizes = ['sm', 'md', 'lg', 'icon', 'icon-sm'] as const satisfies Array<
   ButtonVariants['size']