Adds a 'Canonical' field to the /block page.
note: this requires updating neptune-core in order to obtain the field
from neptune-core's /block_info rpc endpoint.
it was also necessary to change 'use neptune_core::<x>" statements to
'use neptune_cash::<x>'
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
Changes:
1. integrates pico minimal responsive css framework for light/dark
themes and overall better looks.
2. adds a not-found page for bad url queries.
3. catches axum/serde deserialization errors of user input and routes
them to the not-found page with a helpful hint (parse error msg)
