WAIT-AMYBE NOT: Add unallocated direct payments to the dashboard equation

[maebeale]

What is the goal of this PR and why is this important?

• The event dashboard's grand-total equation already surfaces unallocated bulk payments — bulk-payment money received but not yet applied to a registration.
• Direct (non-bulk) payments with a leftover balance (e.g. a registrant who overpaid) were invisible, so the equation understated the cash on hand for the event.
• This adds a parallel Unallocated direct payments term so the dashboard reflects all unapplied money.

How did you approach the change?

• Added EventDashboard#unallocated_direct_payment_cents mirroring unallocated_bulk_payment_cents, and folded it into grand_total_cents.
• Scoped it via a private direct_payments: payments that source an allocation to one of the event's active registrations, with amount_cents_remaining > 0, excluding bulk payments so their remainder is never double-counted.
• Added an amber equation box (same treatment as the bulk term) to events/dashboard.html.erb, shown only when the figure is positive, linking to the payments index filtered to unallocated payments (payments_path(has_remaining: "yes")). Updated the mobile equation label too.
• Added service specs (sum, grand-total inclusion, full-applied/other-event/inactive exclusions, no bulk double-count) and request specs (term renders + links to the filtered index; omitted when fully applied).

Anything else to add?

• Open decision (why this is WAIT): the link target is the global payments index (has_remaining=yes), not event-scoped — the payments index has no event filter, so the list it opens shows unallocated payments across all events while the dashboard figure is this-event-only. The number and the list won't be a perfect 1:1. If we want them to agree, we'd add an event filter to Payment.search_by_params.
• While testing, noted the shared dashboard fixtures set amount_cents_remaining to the full amount on fully-allocated payments; the Allocation after_create callback already decrements that to 0, so values were harmless before but now feed the new figure — left them as realistic full-amount values and let the callback do the math.

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