feat(site,docs): Tane is in F-Droid — link it next to Play

The inclusion MR was merged on 2026-07-25 and the app page went live on
2026-07-28 (v0.1.16, no antifeatures), so the landing page can finally
offer both stores.

- site: publish the F-Droid link and give each store link a single-colour
  icon, so the two are told apart by shape (play triangle vs. the
  free-software robot) rather than by brand colour. The badge row is a
  flex row now: left-aligned in the hero, centred under "Get Tane".
- README: an Install section with both stores, and a Status that matches
  reality (Block 1 shipped, Block 2 under way) instead of "early design".
- release.md: record that the app is published, that further releases only
  need a versionCode bump in the fdroiddata recipe, and that publication
  lags the green build by a day or two.
- open-decisions.md: log what unblocked inclusion — reproducible
  developer-signed builds, a Google-free APK, and opt-in networking.

docs/release.md

@@ -147,11 +147,22 @@ page but does not by itself make the app available in a new country.
 
 ## Publish to F-Droid (official repo)
 
+**Tane is live in F-Droid** since 2026-07-28 (v0.1.16, no antifeatures):
+<https://f-droid.org/packages/org.comunes.tane/>. The inclusion MR
+([43144](https://gitlab.com/fdroid/fdroiddata/-/merge_requests/43144)) was merged
+on 2026-07-25; from now on each release only needs the recipe in `fdroiddata`
+master bumped, not a new inclusion request. Note that publication lags the green
+build by a day or two — check `repo/status/build.json` before assuming breakage.
+
 F-Droid builds from source after a merge request to `fdroiddata`. The build recipe
 is kept in-repo at [`fdroid/org.comunes.tane.yml`](fdroid/org.comunes.tane.yml).
 Copy it to `metadata/org.comunes.tane.yml` in a fork of fdroiddata, validate with
-`fdroid lint` / `fdroid build -l org.comunes.tane`, then open the MR
-([43144](https://gitlab.com/fdroid/fdroiddata/-/merge_requests/43144)).
+`fdroid lint` / `fdroid build -l org.comunes.tane`, then open the MR.
+
+Every version bump **must** advance the recipe's per-ABI `versionCode`s
+(`build × 10 + ABI offset`) together with the `pubspec.yaml` bump — otherwise the
+`fdroid_reference` job fails, and a bad tag cannot be fixed by re-tagging (Play
+rejects a reused versionCode): cut the next version instead.
 
 **Reproducible, developer-signed.** The recipe pins `AllowedAPKSigningKeys` (the
 SHA-256 of the tane-upload certificate) and a `binary:` URL per ABI. F-Droid