build(snap): fill the store metadata

The pack lint flagged five empty metadata fields. They render on the
snapcraft.io listing, and contact is asked for at publish time. The
repo answers for website, source-code, and contact (the issue tracker,
since the listing is public); title matches the binary name.

snap/snapcraft.yaml

@@ -1,6 +1,7 @@
 name: ecrou
 base: core26
 version: "0.1.0"
+title: ecrou
 summary: The strictly confined microVM runtime
 description: |
   ecrou boots disposable Linux environments as one-shot microVMs on
@@ -31,6 +32,10 @@ description: |
   The home interface does not cover dotfiles, so the XDG
   directories are redirected into the snap's common data area.
 license: MIT
+website: https://github.com/vhaudiquet/ecrou
+contact: https://github.com/vhaudiquet/ecrou/issues
+issues: https://github.com/vhaudiquet/ecrou/issues
+source-code: https://github.com/vhaudiquet/ecrou
 grade: stable
 confinement: strict