feat(billing): add direct Stripe billing portal access

Replace dashboard redirect with Stripe Customer Portal session for the
"Manage in Stripe" button, so users go directly to Stripe to manage
invoices, payment methods, and billing details.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/x/packages/shared/src/ipc.ts

@@ -572,6 +572,12 @@ const ipcSchemas = {
       availableCredits: z.number(),
     }),
   },
+  'billing:getPortalUrl': {
+    req: z.null(),
+    res: z.object({
+      url: z.string(),
+    }),
+  },
 } as const;
 
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