Build contacts list view

[strongkeep-debug]

This PR implements Shipyard bounty bnt_6YVRvFJX (Build contacts list view) with a static, no-build contact list UI.

Area	Change	Evidence
Contact list	Adds a table with name, email, company, and last updated columns	index.html, src/main.js
Sorting	Column headers toggle ascending/descending sort state	src/contact-utils.js, src/main.js
Pagination	Adds page-size control, previous/next buttons, and visible range text	src/contact-utils.js, src/main.js
Visual polish	Adds responsive styling and a custom app thumbnail	src/styles.css, public/thumbnail.svg

Validation run locally:

npm test
contact utilities: ok

git diff --check
(no output)