feat: abstract user prompts behind a generic prompt backend trait

[pullfrog]

What changed

• introduced a generic PromptBackend trait in src/tui.rs that abstracts all interactive input operations used by command flows
• added TuiPromptBackend as the default implementation and routed existing prompt helpers through a backend dispatcher
• added with_prompt_backend(...) to temporarily swap prompt implementations for alternate UIs such as GUI integrations
• centralized secret input behind prompt_secret_non_empty(...) and updated encrypt passkey confirmation to use the backend abstraction

Why

This allows the same command flows to run with TUI prompts today and makes it straightforward to plug in GUI prompt implementations without duplicating flow logic.

Verification

• cargo fmt
• cargo check
• cargo test prompt_char_tests

  ^{｜ View workflow run ｜ Triggered by Pullfrog ｜ Using GPT Codex ｜ 𝕏}

[pullfrog]

Addressed the requested review changes on PR #499 by moving PromptBackend into a new module and redesigning it around GUI friendly abstractions (prompt_text, choose, select_confirmations, prompt_secret) while removing pause from the trait. Pushed commit fb9c608 to pullfrog/0-abstract-prompt-backend, replied to both review comments, and resolved both threads. Validation passed with cargo check and cargo test prompt_char_tests.

Task list (6/6 completed)

• Checkout PR feat: abstract user prompts behind a generic prompt backend trait #499 branch and inspect reviewer feedback
• Implement requested code changes for each review comment
• Run quality checks and review diff
• Commit and push branch with clean working tree
• Reply to review comments and resolve addressed threads
• Post final progress update

  ^{｜ View workflow run ｜ Triggered by Pullfrog ｜ Using GPT Codex ｜ 𝕏}