Improve keyboard navigation: uppercase Q quit and more-style paging

[can2049]

Summary

• Accept uppercase Q as well as lowercase q to quit reading mode
• Keep Ctrl+Q consistent with Ctrl+q for opening the fuzzy file picker
• Add more-style paging: Space pages down, b pages up (same step as d/u / PgDn/PgUp)
• Update help popup and status bar hints (q/Q quit, u/d spc/b)
• Sync docs: README keybindings, Mermaid demo, and navigation demo

Motivation

• Quit with Q: With a Chinese IME, switching to type lowercase q usually means leaving Chinese input first. With Caps Lock on, pressing Q exits without changing the input method.
• Space / b paging: Matches familiar more/less habits so one-handed paging is easier while reading.

Test plan

• cargo build (or cargo run -- README.md)
• In reading mode, press q — app exits
• In reading mode, press Q (or Caps Lock + Q) — app exits
• In reading mode, press Space — page down (~20 lines)
• In reading mode, press b — page up (~20 lines)
• Confirm help (?) shows q/Q quit and u/d spc/b
• Confirm status bar shows q/Q quit
• Confirm README lists q/Q, Space, and b
• Confirm Ctrl+q / Ctrl+Q still open the fuzzy file picker

[RivoLink]

Hi, thanks for your PR!
I was just wondering if you have a specific use case that requires shift+q?

[can2049]

Yes — with a Chinese IME, switching to type lowercase q usually means leaving Chinese input first. With Caps Lock on, pressing Q exits without changing the input method.

[RivoLink]

Hi,
I created a dedicated PR for your request: #216.
I made it a hidden shortcut because it's an extra feature that most users won't need.

I preserved you as the author and signed the commit with my GPG key, since GPG signing is required before merging into the main branch.

If you'd like to learn more about GPG commit signing, you can find the documentation here:
https://docs.github.com/en/authentication/managing-commit-signature-verification/signing-commits

Thanks again for your interest in leaf and for your contribution!