Add trial days remaining to billing card, 5-min polling, and wire upgrade button

- Add trialDaysRemaining to IPC schema, useBilling hook, and BillingInfo interface
- Show trial countdown in sidebar billing card (e.g. "7 days left on trial")
- Add 5-minute polling interval for billing data refresh
- Wire Upgrade button to open web billing page in system browser

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

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

@@ -564,6 +564,7 @@ const ipcSchemas = {
       userId: z.string().nullable(),
       subscriptionPlan: z.string().nullable(),
       subscriptionStatus: z.string().nullable(),
+      trialDaysRemaining: z.number().nullable(),
       sanctionedCredits: z.number(),
       availableCredits: z.number(),
     }),