Mailroom landing page

[spacebear21]

Closes #1395

Authored by Claude Opus 4.6 and several attempts at fine-tuning the copy and design. Final result:

Pull Request Checklist

Please confirm the following before requesting review:

• I have disclosed my use of
  AI
  in the body of this PR.
• I have read CONTRIBUTING.md and rebased my branch to produce hygienic commits.

[coveralls]

Pull Request Test Coverage Report for Build 22881193304

Details

• 8 of 13 (61.54%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.3%) to 83.844%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
payjoin-mailroom/src/directory.rs	8	13	61.54%

Files with Coverage Reduction	New Missed Lines	%
payjoin-mailroom/src/directory.rs	1	85.04%

Totals
Change from base Build 22866682076:	0.3%
Covered Lines:	10696
Relevant Lines:	12757

---

💛 - Coveralls

[caarloshenriq]

Tested locally — ran the branch and the landing page loaded correctly. Great work!

One suggestion: it would be nice to add a test to ensure the placeholders are always replaced correctly before the HTML is served:

    #[test]
    fn landing_page_contains_version() {
        let html = landing_page_html();
        assert!(!html.contains("{{VERSION}}"));
        assert!(!html.contains("{{COMMIT}}"));
    }

This guards against any future refactor accidentally leaving a raw {{VERSION}} or {{COMMIT}} in the response.

[spacebear21]

Great suggestion, thanks @caarloshenriq

[bc1cindy]

you gotta pretty and modern mailroom @spacebear21 , it matches payjoin standards!

tested locally and it's running perfectly, with all external links working well