release: open days older than the first entry ever logged (#4)

## Fixed

- Days older than the first entry ever logged can now be opened. Picking a
  date from before anything was recorded landed on a dead end.

## Changed

- Releases are built by CI. Merging this pull request builds the image,
  tags it `vYYYYMMDD-N` and `latest`, pushes both, and creates the git
  tag. Nothing is built locally any more.
- `make image`, `push`, `release`, `seed`, `icons` and `vendor` are gone;
  the Makefile is down from 151 lines to 71. The two rare commands are
  written out in the README.
- Every push to `dev` now runs `make check` in CI.

## Deployment

No new or renamed environment variables, and `compose.yaml` is unchanged —
nothing to copy to the server this time. Pull the new image once CI reports
the build finished.

---------

Co-authored-by: Esa Kataja <[email protected]>
Reviewed-on: #4

.gitignore

@@ -4,9 +4,6 @@
 # Build output
 /foodster
 
-# The tag `make image` last built, handed to `make push`.
-/.release-tag
-
 # Generated by `templ generate` during the container build.
 *_templ.go