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/assets/css/main.css

@@ -219,7 +219,14 @@ h2 { font-size: clamp(1.4rem, 3vw, 1.9rem); color: var(--green-dark); }
 .values li { margin: .45rem 0; }
 
 .get-it { text-align: center; }
-.store-badges { margin-top: 1.2rem; }
+.store-badges {
+  margin-top: 1.2rem;
+  display: flex;
+  flex-wrap: wrap;
+  gap: .6rem;
+  justify-content: inherit;
+}
+.get-it .store-badges { justify-content: center; }
 .coming-soon { color: var(--muted); font-style: italic; }
 .badge {
   display: inline-block;
@@ -231,6 +238,14 @@ h2 { font-size: clamp(1.4rem, 3vw, 1.9rem); color: var(--green-dark); }
   margin: .3rem;
   font-weight: 600;
 }
+/* Store links: same pill, one-colour icon so Play and F-Droid read apart. */
+.badge-store {
+  display: inline-flex;
+  align-items: center;
+  gap: .55rem;
+  margin: 0;
+}
+.badge-store .store-icon { flex: none; }
 
 /* Legal docs */
 .doc { padding: 2rem 0 3rem; max-width: 760px; }