feat(ui): the M of the wordmark is the logo

The stylised M replaces the letter M in "MeshBay" at the top left; `eshBay`
beside it stays text.

**The source artwork is now in git.** It was in `QE/`, which is gitignored, so
it would not have survived a clone — and the tree is about to be moved to
another machine. `assets/brand/` holds the original and the `convert` command
that derives the 10 KB nav asset from it (trimmed, 72px tall for a picture shown
at 30, so it survives a 2× screen). Verified: the documented command reproduces
the committed file byte for byte.

The derived file is in `_ASSETS`, which is what the `/a/<hash>/` fingerprint is
computed from. A file missing from that list is a file whose change never moves
the URL, so a browser holding the old one never asks again — the comment on that
list says so, and a logo is exactly the kind of asset one would forget.

Its URL is resolved from `import.meta.url`, so the hub's fingerprinted path and
the application's `app://` scheme both come out right without either being named
in the source.

**The two sizes are independent, and that took four rounds to get right.** The
picture's height was written in `em` — a fraction of the lettering — so every
adjustment to the text resized the picture by the same stroke and the ratio
between them could never change. The operator spotted the coupling before I did
("si on change la taille du M, tu vas encore tout décaler"). The height is in
pixels now: `font-size: 1.375em` (22px) and `height: 30px`, two knobs that move
one thing each. The vertical nudge went with it — it was correcting a
misalignment I was causing myself.

Also: the wordmark blue goes one step down from sky-400 towards sky-500. It sat
brighter than the picture next to it, which made the two read as different
materials rather than one object.

A note on how this went wrong first: I checked the result on a mock HTML page I
wrote, not in the application. The picture was loading the whole time, but at
25px against a 29px line box it was *smaller* than the letters, so the wordmark
read as unchanged and the report was "none of what I asked for was done". Every
size since has been measured in the running window over CDP.

883 tests pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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