merge: reconcile master particle-wind work with origin/main redesign

Merged divergent branches (common ancestor a9d87b6). Took origin/main
as base (layer manager, recenter, quit button, GPX tracks, marine
conditions, dark theme, user position layer, active/past track layers)
and layered in particle wind simulation work:

- MapHandler: added setupWindLayer, updateWindLayer, updateWindGridLayer;
  kept their user-pos layer, follow/recenter state, active+past tracks
- WeatherRepository: kept their fetchCurrentConditions, added fetchWindGrid
- MainViewModel: kept their track/marine/summary changes, added windGrid
  StateFlow, loadWindGrid, buildGrid
- MainActivity: kept their full UI (layer picker, quit, recenter, fade
  animations), added particleWindView wiring, weatherLoaded guard,
  windArrow/windGrid observers
- activity_main.xml: took their LinearLayout redesign, added ParticleWindView
  overlay inside ConstraintLayout

https://claude.ai/code/session_01HXPjBsogsJVRwCiekDGkJX

1 files changed, 9 insertions, 0 deletions

diff --git a/android-app/app/src/main/res/drawable/ic_layers.xml b/android-app/app/src/main/res/drawable/ic_layers.xml
new file mode 100644
index 0000000..f86f83a
--- /dev/null
+++ b/android-app/app/src/main/res/drawable/ic_layers.xml
@@ -0,0 +1,9 @@
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    android:width="24dp"
+    android:height="24dp"
+    android:viewportWidth="24"
+    android:viewportHeight="24">
+    <path
+        android:fillColor="?attr/colorOnSurface"
+        android:pathData="M11.99,18.54L4.62,12.81 3,14.07l9,7 9,-7-1.63,-1.27-8.38,5.74zM12,16l8.36,-6.54L22,8.07l-10,-7-10,7 1.63,1.39L12,16z"/>
+</vector>