Rewrite dev setup docs with tested, copy-pasteable instructions

Replaces the vague "Latest stable version" prerequisites in DEVELOPER.md
with concrete commands tested on a fresh macOS machine. Adds Android SDK
setup without Android Studio, documents the gen_icons_splashes.sh
requirement, and fixes the OAuth2 config to reference build_keys.conf
instead of the removed keys.dart.example. Also includes Flutter SDK
auto-migrations (iOS 13.0 min, gradle minSdk).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ios/Flutter/AppFrameworkInfo.plist

@@ -21,6 +21,6 @@
   <key>CFBundleVersion</key>
   <string>1.0</string>
   <key>MinimumOSVersion</key>
-  <string>12.0</string>
+  <string>13.0</string>
 </dict>
 </plist>