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

assets/icon.svg

@@ -1,6 +1,6 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512" width="512" height="512"
      role="img" aria-label="Foodster">
-  <!-- Source for the home-screen PNGs; `make icons` rasterises it.
+  <!-- Source for the home-screen PNGs; the README says how to rasterise it.
        Unlike favicon.svg this one is opaque and fixed-colour: a home screen
        icon cannot be transparent and must not change with the system theme.
        The bowl sits inside the central 80% so Android can mask it to any