Fix null Stripe invoice crash in upgrade preview

[simonhamp]

Summary

Fixes #63: Error: Call to a member function asStripeInvoice() on null thrown from app/Livewire/MobilePricing.php:121 when a user opens the upgrade modal.

Root cause

Cashier's Subscription::previewInvoice() returns null when Stripe has no upcoming invoice (canceled subscription, missing customer, etc.). The existing try/catch (\Exception) didn't catch the resulting \Error (PHP Error doesn't extend Exception), so it surfaced as a 500.

Fix

• Added a null guard after previewInvoice() so the existing "Unable to load pricing preview" fallback UI renders gracefully.
• Widened the catch from \Exception to \Throwable as a defense-in-depth for any future PHP errors on this path.
• Added a degraded preview branch for canceled-but-in-grace subscribers (most importantly EAP folks who legitimately want to upgrade). For that state, Stripe returns no upcoming invoice, so we skip the Stripe call entirely and build the preview from configured plan pricing. The modal shows the new charge with a "(pro-rated)" tag and a line clarifying that the remaining time will be credited at checkout. Stripe still computes the actual proration server-side at swapAndInvoice time, so the charge stays correct.

Test plan

• New test reproduces the exact production error when the fix is reverted (preview_upgrade_sets_preview_to_null_when_stripe_has_no_upcoming_invoice).
• New tests cover the degraded-preview path (canceled-in-grace user gets a preview without touching Stripe; EAP customers see the EAP yearly price; modal renders the pending-proration copy).
• All 30 MobilePricingTest cases pass; broader Livewire/Stripe test sweep (185 tests) passes.
• Manually verify in staging: open the upgrade modal as a canceled-in-grace EAP user.

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