Fix #633-#636: DeadlineSuggester tooltip, gauge zones, hook split, wallet retry

[lawrencesunday311-sketch]

Title

Fix #633-#636: DeadlineSuggester tooltip, gauge zones, hook split, wallet retry

Body

This PR addresses four small frontend issues:

#633 — DeadlineSuggester reasoning tooltip. The recommended-deadline pill now includes a keyboard- and mouse-accessible "?" tooltip that explains why the suggestion was made (average funding time from similar past invoices, or the static fallback reasoning when no history is available).

#634 — PaymentVelocityGauge performance zones. Each gauge arc now renders red (0-33%), yellow (34-66%), and green (67-100%) background zones so users get an instant visual read on velocity, with zone boundaries configurable via an optional zoneBoundaries prop. The value arc and labels render on top of the zones as before.

#635 — Split useInvoiceForm. Extracted useLineItems (item CRUD, totals, reordering) and useInvoiceMeta (expiry date/timezone fields and validation) into their own files. useInvoiceForm now composes both and re-exports the same unified API, so no call sites needed changes.

#636 — useWalletBalance retry with backoff. On a failed balance fetch, the hook now retries up to 3 times with 1s/2s/4s delays, exposing an isRetrying flag (surfaced as a "Retrying..." indicator in WalletBalanceDisplay). The error state is only shown once retries are exhausted, and a successful retry clears it.

Closes #633
Closes #634
Closes #635
Closes #636

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