Keep the landing announcement banner readable on phones

The announcement banner on the landing page never wraps: .announce sets white-space: nowrap, overflow: hidden and text-overflow: ellipsis. The v0.2.0 copy is 861px wide, so a phone sees only the start of it. The review of v0.2.0's site measured "NEW Headroom v0.2.0 — quiet-n…" at 320px and "…quiet-node fixes, work…" at 390px. "Read the announcement →" is cut off below about 861px. The page itself does not scroll sideways. The v0.1.0 banner had the same shape, so the bug is in the banner's style rather than this release's copy. Q64 (tenants.html width) and Q65 (the docs navbar) do not cover it.

Let the banner wrap below a breakpoint, or show a shorter phone text such as "v0.2.0 is out →". Check at 320px and 390px.