gui: Use concise Bitcoin Core wallet-encryption guidance #73

Tails 7.13 field-test finding: replace implementation-focused wallet-encryption copy with concise operator guidance about what Bitcoin Core wallet encryption protects, the consequence of forgetting the passphrase, and the role of the codex32 recovery cards. Preserve the existing no-argv/no-file passphrase transport boundary and do not add a local password policy.

Implemented in focused GUI PR #77 on top of #65/#66. Exact-head run 464 is green. Keep this issue open until the final GUI stack is rebased onto the settled library/CLI candidate and #77 is integrated; the final manual GUI qualification should confirm the copy remains readable at the supported Tails guest resolution.