Add standalone wiring diagram PNG export

Fetch the wiring diagram from the VoltPlan API and share it as a
standalone PNG from the Overview share menu on both platforms, with a
localized error when the diagram cannot be generated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

android/app/src/main/res/values/strings.xml

@@ -5,6 +5,7 @@
     <!-- Actions -->
     <string name="action_add">Add</string>
     <string name="action_back">Back</string>
+    <string name="action_save">Save</string>
     <string name="action_delete">Delete</string>
 
     <!-- Systems -->
@@ -177,6 +178,8 @@
     <string name="overview_chargers_empty_subtitle">Add shore power, DC-DC, or solar chargers to understand your charging capacity.</string>
     <string name="overview_chargers_empty_create">Add Charger</string>
     <string name="overview_share_pdf">Full Report (PDF)</string>
+    <string name="overview_share_diagram">Wiring Diagram</string>
+    <string name="overview_share_diagram_error">Could not generate diagram. Check your internet connection.</string>
 
     <!-- Goal editor steppers -->
     <string name="goal_days">Days</string>