Use transaction uid for invoice download

[TaprootFreak]

Summary

• Change invoice button to use tx.uid instead of tx.id
• Remove hidden condition - button is now always visible
• Update isInvoiceLoading state type from number to string

Why

Pending transactions (e.g., "Warten auf Zahlung") don't have a numeric id, only a uid. This change enables invoice download for all transactions.

Dependencies

This PR requires the following PRs to be merged first:

1. API: Allow invoice generation for pending transactions via uid api#3029 - Backend support for uid in invoice endpoint
2. Packages: Support uid parameter for getTransactionInvoice packages#124 - Type update for getTransactionInvoice

Temporary workaround

Contains a type cast (tx.uid as unknown as number) that should be removed after @dfx.swiss/react package is updated.

Test plan

• Lint passes
• Build passes
• After dependencies merged: verify invoice download works for pending transactions

[github-actions]

🤖 PR Review Bot

ℹ️ New TODOs/FIXMEs (1)

+                                // TODO: remove cast after @dfx.swiss/react update

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