fix(ui): give the tag, user and badge pages an h1

[culfin]

Follow-up to #1588, which fixed the question list and the two pages that embed
it. Three more pages carry their title as h3 and have no h1 at all:

ui/src/pages/Tags/index.tsx:84    <h3 className="mb-4">{t("title")}</h3>
ui/src/pages/Users/index.tsx:46   <h3 className="mb-4">{t("title")}</h3>
ui/src/pages/Badges/index.tsx:38  <h3 className="mb-4">{t("title")}</h3>

The groups below them jump further than one level — h6 on the user page,
h5 on the badge page. Those become h2, one step under the title.

file	before	after
Tags/index.tsx	h3 title	h1
Users/index.tsx	h3 title, h6 groups	h1, h2
Badges/index.tsx	h3 title, h5 groups	h1, h2

fs-3, fs-6 and fs-5 carry the sizes the old tags had, so nothing
changes on screen. Checked with prettier, eslint and tsc.

Still open

The settings pages and the search results have no heading at all — not a wrong
level, none. Adding one means choosing wording and adding an i18n key across 45
language files, which is a different kind of decision than renaming a tag, so
it is not in here.

🤖 Generated with Claude Code

[culfin]

Folded into #1588, which now covers these pages as well. Nothing is dropped; the commit is on that branch. Closing to keep the queue readable.