Fix FeeOptimizer, NotificationCenter, ReminderSender, and CountdownTimer

[deeejaymu-afk]

Title

Fix FeeOptimizer, NotificationCenter, ReminderSender, and CountdownTimer

Body

This PR addresses four small UX issues in the invoice components:

FeeOptimizer now shows an "Accept Suggestion" button next to the recommended fee. Clicking it applies the suggested fee (via an onAcceptFee callback for callers that manage a transaction fee input, with an internal fallback so the button behaves correctly standalone). The button hides itself once the current fee already matches the suggestion.

NotificationCenter already had a "Mark all as read" button; it now performs an optimistic update immediately and awaits persistence, rolling the UI back if persistence fails, matching the "optimistic update + persisted via the notifications API" requirement.

ReminderSender gains an optional "Send at" datetime input. Leaving it empty preserves the existing immediate-send behavior. Choosing a future time enables a "Schedule Reminder" action that stores the scheduled reminder and disables the immediate-send actions; choosing a past time shows an inline validation error.

CountdownTimer (DeadlineCountdown) already clamped elapsed time at zero and cleared its interval on expiry; it now also accepts a configurable expiredLabel prop (defaulting to "Expired") instead of a hardcoded string, and stops re-rendering once expired.

Closes #629
Closes #630
Closes #631
Closes #632

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