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.

site/config.toml

@@ -16,7 +16,7 @@ enableGitInfo = false
   license = "AGPL-3.0"
   # Links rendered only when set, so nothing broken shows before they exist:
   playStoreURL = "https://play.google.com/store/apps/details?id=org.comunes.tane"   # Google Play
-  fdroidURL = ""      # F-Droid, once published
+  fdroidURL = "https://f-droid.org/packages/org.comunes.tane/"      # F-Droid, published 2026-07-28
   sourceURL = "https://git.comunes.org/comunes/tane"      # public source repository (AGPL)
   weblateURL = "https://translate.comunes.org/projects/tane/"     # translation platform