Add alt-screen and key bindings to live mode

[Tairesh]

Summary

Live mode now uses the terminal's alternate screen buffer with non-blocking key handling, so the user's existing terminal contents are preserved and they can quit or refresh interactively.

• Alt screen: enters via crossterm on first successful fetch, restored on exit (clean exit, Ctrl+C, error path, panic) by a TerminalGuard RAII type.
• Keys: q / Esc / Ctrl+C quit immediately; r forces an immediate refresh.
• Footer: optional one-line footer with key hints + last-update timestamp ([q] quit [r] refresh • last update: HH:MM:SS). Configurable via live_mode_footer = true|false in TOML and --no-footer on the CLI.
• First fetch outside alt screen: keeps the user's normal terminal visible while the initial API call loads instead of showing a blank alt screen.
• Provider-fallback refactor: App::fetch_with_fallback is now a method, used by both one-shot and live paths. The live path can now return Result so TerminalGuard runs before any error exit (previously display_error called process::exit, skipping destructors).

Test plan

• just check passes — 104 tests, clippy -D pedantic clean
• New regression tests for live_mode_footer config field (defaults, explicit false, CLI override, CLI absent preserves config value)
• New tests for format_footer (color on/off) and write_payload (CRLF translation, 5 edge cases)
• New tests for WeatherFormatter::render_to_string (full text + one-line modes)
• Manual smoke test on Linux: cargo run -- -c "<city>" --live -i 10 — verify alt-screen restore, key bindings, footer
• Manual smoke test with --no-footer
• Manual smoke test on macOS / Windows (nice-to-have)

Notes

• Adds a single dependency: crossterm = "0.28" (cross-platform alt-screen + raw mode + key events; no separate ctrlc crate needed since Ctrl+C arrives as a regular KeyEvent in raw mode).
• One-shot path (rustormy -c <city>) is byte-for-byte identical to before — only the live path changed.
• \n → \r\n translation is required because raw mode disables the kernel's ONLCR line-discipline; verified to behave the same on Linux, macOS, and Windows console.

[gemini-code-assist]

Code Review

This pull request implements a robust live mode for the weather application using the crossterm library, featuring an alternate screen buffer, an interactive event loop, and a customizable footer with update timestamps. The core application logic was refactored to separate weather data fetching and rendering, allowing for continuous updates and better error handling with provider fallbacks. Review feedback highlights opportunities to optimize performance by passing weather data by reference instead of by value and suggests improving the event loop to handle terminal resize events to prevent display corruption.