Add BIP39 word-number mnemonic entry

[oroderico]

Summary

• add an optional BIP39 word-number entry method for 12- and 24-word mnemonic recovery
• resolve numbers from 1 to 2048 to their English BIP39 words and require confirmation before continuing
• keep the standard Begin Setup flow unchanged, exposing the Entry Method screen only in Advanced Setup and Temporary Signer
• preserve the existing mnemonic validation and advanced final-word behavior

This implements the flow agreed in #311, including the Entry Method title and the Word / Index Number options. Thanks to @jgriffiths for routing the proposal to the product team, and to @bitcoinhelp for the product feedback and confirmation of the updated flow.

Closes #311.

Implementation

• extend digit entry with bounded, variable-length numeric input while preserving existing PIN and index-entry behavior
• share the existing word-selection and final-word logic between word and index-number recovery
• securely clear replaced GUI text and temporary mnemonic-entry state

Testing

• validated 12-word and 24-word recovery by index number
• validated standard Begin Setup, Advanced Setup, and Temporary Signer routing
• validated word confirmation, backtracking, invalid numbers, and invalid mnemonic handling
• validated existing and calculated advanced final-word paths
• built, flashed, and tested on a TTGO T-Display
• passed clang-format --dry-run --Werror and git diff --check