Use the rodent emblem as the logo and the favicon

Use the rodent emblem as the logo and the favicon

The header carried a letter in an accent chip and the favicon was a separate "v"
glyph, so the forge had two marks that could drift apart. They are now one piece of
artwork, served from the binary like every other static asset: a dark rounded tile
with a white rodent face, named by the shell as both the icons and the header mark.

The source drawing is a tile on a white page. Only the page is removed — a fuzz
flood fill from the corner, which leaves the face opaque because the tile encloses
it — so the mark has transparent corners and reads on either theme without a second
set of assets. Sizes are 128 for the header (22 CSS px, so it survives a high-DPI
screen), 32 and 16 for the tab, each square and centred because a favicon that is
31 pixels tall is a favicon some browser will letterbox.

favicon.svg is removed; nothing else referenced it. A test now asserts that the
three files are served as PNG and that the shell names them, so the next rename
cannot leave every page pointing at a missing icon.