PM-39072: Feat: Update premium action card copy

[david-livefront]

🎟️ Tracking

PM-39072

📔 Objective

This PR updates the premium action card on the VaultScreen to use new copy.

📸 Screenshots

Before	After

[github-actions]

Code Review: PM-39072 — Update premium dialog copy

Reviewed the change updating the UpgradePremium action card button on the Vault screen from "Upgrade to Premium" to "Learn more", plus the two corresponding test assertions.

Summary

✅ Looks good. This is a clean, well-scoped copy change.

• The learn_more string resource exists in the :ui module (strings.xml:214).
• Both affected VaultScreenTest assertions were updated to match the new copy, keeping the click-behavior test intact.
• The change is correctly limited to the UpgradePremium action card. The upgrade_to_premium string remains in use across other premium-gating dialogs (Send, Attachments, Vault Item, etc.), which is expected and outside this ticket's scope.
• Existing behavior is preserved: onActionClick still routes through actionCardClick → NavigateToUpgradePremium → onNavigateToPlan(), so only the label changed.

Notes

• The sibling UpgradedToPremium card already uses the same learn_more label but with isExternalLink = true. The two cards are mutually exclusive states, so no UX ambiguity is introduced; the differing isExternalLink is consistent with each card's distinct navigation target (internal plan screen vs. external link).

No blocking issues, no suggestions. 👍

[david-livefront]

Thanks @LRNcardozoWDF