Previously the logo looked very jagged against a dark background.
This makes it look smooth for dark theme. However it is a bit blurry
against white background.
Hopefully can replace with a higher quality graphic soon.
Changes:
+ present not-found page for unhandled uri paths, eg /stuff
+ display the tip-height on front page
+ add favicon and neptune logo to front page
+ use ServerDir instead of ServeFile for /image and /css
+ add help tooltips for fields on front page
+ use monospace font for digests
