web: mobile Explore, registration key help, empty search !459

The three findings from the external review that held up on inspection. The rest are recorded in #248 with why they were not taken.

• Explore is a rail square at every width. It was .railopt, so below 34rem it dropped into the More menu, which renders only for a signed-in viewer: a signed-out phone had no route to the listing from any page. Seven targets at 44px are 308px, verified at 320px with no horizontal overflow. TestSignedOutRailDropsNothing states the invariant — a signed-out rail marks nothing .railopt.
• /register answers "no key yet?" with a disclosure holding ssh-keygen and cat, instead of a link that left the page for the wiki over a half-filled form. The link also hardcoded gitbay.org.
• An empty /search says what to do next rather than repeating the count line's "no matches".

The CHANGELOG section is written as v1.34.1; rename it if these ship with something larger.

axe over /register, /search with no hits and /explore at 1280 and 320: zero violations, no target under 24px.