[smaranjit]

What does this change?

Adds a short, one-time request for a GitHub star in four places:

• README.md: a line above Contributing.
• User guide front page (docs/src/introduction.md): at the end, after Where to start.
• README.txt: which ships inside every download, in the MORE section.
• About dialog (src/ui/about.rs): "Find it useful? Star it on GitHub — it really helps." The link goes to the repository, using CARGO_PKG_REPOSITORY like the existing Source link. It lives only in About, never on the main screen.

The guide's About screenshot is refreshed to match, and the CHANGELOG has an [Unreleased] entry.

Checklist

• cargo fmt --all -- --check passes
• cargo clippy --all-targets -- -D warnings passes
• cargo test --all-targets passes
• cargo build --release succeeds
• Tested manually against a Firebase project (a development one): the About dialog renders the new line without changing its size
• CHANGELOG.md updated under [Unreleased], if user-facing

Notes for the reviewer

The wording asks only for a GitHub star. The app isn't on the Microsoft Store, so a Store review request would point at a listing that doesn't exist. The About-dialog change reaches users with the next release.