Add documentation on F-Droid publishing requirements and considerations

Author: MartinBraquet

docs/fdroid.md

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+# F-Droid publishing
+
+Can Compass be published on [F-Droid](https://f-droid.org/)? Short version: **licensing allows it, but the
+Google/Firebase dependencies block the official repo.**
+
+## Eligibility
+
+- **License** — ✅ AGPL-3.0 + MIT. Meets F-Droid's hard FOSS-license requirement.
+- **Buildable from source** — feasible; it's a Capacitor WebView shell over the `web` build.
+- **Backend** — ⚠️ the app wraps the hosted service (compassmeet.com, Supabase + Firebase). F-Droid tags
+  this `NonFreeNet` ("depends on a non-free network service") regardless of the client code.
+
+## Blocking dependencies
+
+Proprietary Google libraries F-Droid's main repo bans. From `android/app/build.gradle`:
+
+- `com.google.firebase:firebase-analytics` — proprietary **and** tracking; an outright anti-feature.
+- `com.google.firebase:firebase-auth` — proprietary Google auth.
+- `com.google.android.gms:play-services-auth` — proprietary Google Play Services.
+- `com.google.android.play:app-update` / `app-update-ktx` — proprietary in-app-update lib.
+- `com.google.gms.google-services` Gradle plugin + `google-services.json` — non-free build tooling/config.
+
+From the web bundle (`web/package.json`):
+
+- `firebase` (11.1.0) — proprietary JS SDK compiled into the WebView assets.
+- `@capacitor/push-notifications` — wired to Firebase Cloud Messaging (FCM), which is proprietary.
+
+## Paths to ship
+
+**Official F-Droid repo** — significant work:
+
+- Remove `firebase-analytics` (or swap for self-hosted Matomo/Plausible).
+- Replace Firebase Auth with a Supabase-side / FOSS auth flow.
+- Replace FCM push with **UnifiedPush** (the F-Droid-blessed push system) — the biggest lift.
+- Drop the in-app-update lib (F-Droid handles updates).
+- Remove `google-services.json` and the gms plugin.
+- Add F-Droid metadata + a reproducible build recipe. A `NonFreeNet` label likely remains because the
+  backend is a hosted service.
+
+Estimate: days-to-weeks, dominated by the auth + push replacement.
+
+**Self-hosted F-Droid repo** — easy:
+
+- Use `fdroidserver` to host the existing signed APK/AAB at your own repo URL (Firebase and all). The
+  anti-feature bans are official-repo _policy_, not enforced by the tooling. Users add the repo manually
+  and get auto-updates; you skip Google's storefront. Setup is roughly an afternoon.