refactor(metassr-html): idiomatic From impls, simplified Display, and modernized builder bounds

[abhicodes0324]

Summary

Four idiomatic Rust improvements to metassr-html. No behaviour changes - all existing tests pass and 8 new tests added.

Changes

• HtmlOutput: Replace custom pub fn from(html: &str) with impl From<String> + impl From<&str>. From<String> moves the value in zero-copy; the old method shadowed the trait without implementing it.
• Display: Simplify f.write_fmt(format_args!(…)) → write!(f, …).
• generate(): Fix double allocation - From<String> now takes the owned format!("{}", self.template) directly instead of borrowing a .to_string() intermediary.
• HtmlPropsBuilder: Change setter bounds from &S: ToString + ?Sized to impl Into<String>. Accepts &str, String, and Cow<str> at call sites without explicit conversion and moves owned Strings without cloning. Also replace unwrap_or(&String::new()).to_owned() with unwrap_or_default().

Checklist

• No behaviour changes
• All existing tests pass
• 8 new unit tests added
• cargo clippy clean
• cargo fmt applied

[abhicodes0324]

Hi @fahdfady @hulxv, please have a look when you get time.