Web dashboard step 7: docs, users page, polish

Users page, README and config.example updates, implementation notes, the
rollout runbook, site-layout 404/500 pages, human-readable download sizes,
dark-mode and narrow-screen polish, and a smoke test over every dashboard
route; also removes zdiff3 ancestor markers left by earlier merges.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NHyYupFdBiR4VfoUM7NjSM

src/web/static/app.js

@@ -56,7 +56,7 @@ document.querySelectorAll("table[data-filter]").forEach((table) => {
       row.hidden = needle !== "" && !row.textContent.toLowerCase().includes(needle);
     });
   });
-||||||| 849231e
+});
 /* step 5: settings — "reset to default" fills the field with its default */
 document.addEventListener("click", (event) => {
   const button = event.target.closest("button[data-reset]");