Dashboard routes: add centre, encode topic/<file>

tsk-ncpxvh8hq5s2 open

Goal

centre is part of the routes grammar: routes.format/routes.parse round-trip it, and topic/<file> segments are encoded.

Why

Today format/parse drop centre, so a shared link loses the centred node; unencoded topic/<file> breaks on reserved characters. Found in the maity#1286 M3 review by the integrator.

Scope

Add centre to the grammar in both directions, documented next to the existing route keys.
Encode/decode topic/<file> segments.
Round-trip tests incl. files with spaces, #, ?, %, /.

Release

Next dashboard release after 0.11.43, on Oliver's go.