Release signing fix + Obtainium install guide + publish script

- Fix withReleaseSigning: RN template's unconditional debug signingConfig was
  overriding the release key. Now uses the release key when BBP_UPLOAD_* is set
  (verified: APK signed CN=BigBrainParking), else debug.
- INSTALL.md: dead-simple Obtainium setup (one-tap obtainium:// link + manual
  Forgejo-source steps). README links it.
- scripts/publish-release.sh: create a Forgejo release + upload the signed APK.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

INSTALL.md

@@ -0,0 +1,46 @@
+# Install BigBrainParking
+
+BigBrainParking is distributed through **Obtainium** — a free, open-source app updater.
+No Google Play, no account. Works great on **GrapheneOS** and any Android phone.
+
+## 1. Get Obtainium (once)
+
+Install Obtainium from **F-Droid** or its GitHub releases:
+- F-Droid / IzzyOnDroid: search "Obtainium"
+- or https://github.com/ImranR98/Obtainium/releases (grab the `.apk`)
+
+## 2. Add BigBrainParking
+
+**Easiest — one tap:** open this link on your phone (with Obtainium installed):
+
+> `obtainium://add/https://git.mowden.top/hank/BigBrainParking`
+
+It opens Obtainium's Add screen pre-filled. Tap **Add**.
+
+**Manual (if the link doesn't work):**
+1. Open Obtainium → **Add App**.
+2. Paste the source URL:
+   ```
+   https://git.mowden.top/hank/BigBrainParking
+   ```
+3. If asked for a source type, choose **Forgejo** (git.mowden.top runs Forgejo).
+4. Tap **Add**.
+
+## 3. Install & update
+
+Obtainium shows the latest version — tap **Install**. GrapheneOS/Android will ask you to
+allow installs from Obtainium the first time; approve it.
+
+From then on, Obtainium notifies you when a new version is released and updates in a tap.
+
+## Notifications (optional)
+
+Parking-expiry reminders work on-device with **no Google services**. For any future
+server-pushed notifications, install **ntfy** (F-Droid) and set it as your UnifiedPush
+distributor — BigBrainParking will use it automatically.
+
+## Notes
+
+- Unofficial client for the ParkSmarter (IPS Group) parking system. Not affiliated with or
+  endorsed by IPS Group. Use with your own ParkSmarter account.
+- Source & releases: https://git.mowden.top/hank/BigBrainParking