introduces the filetree package.

eventually we will have a sticky side-panel style layout for any page
displaying diffs (probably makes most sense when we have split diffs),
and this file-tree will move there.

in dev mode; all html templates are loaded from disk on access. UI
updates take <100ms now:

- go files are watched by air/gust
- html/css files are watched by `tailwind -w`

html/css changes do not cause appview reload; but do trigger reload of
static assets and templates; giving us instant live reloads for UI
development.

follows our diff fragment colorscheme now

and move appview/state/settings to appview/settings

move settings router into a subpackage

also introduces a middleware package under appview, and turns TID() into
a global function that operates on a globally mutable TID clock.

PRs are no longer merged as a single commit.