feat: apply Charbon·Lime design system, add gear icon, multi-provider git settings

- Replace app icon with new Charbon·Lime brand assets (icon.png + adaptive-icon.png)
- Apply design tokens to theme.ts: bg #1B2433, primary/success #88D656 (lime), text1 #EEF2F5 (cream)
- Add ⚙ gear button (lime) in HomeScreen header → navigates to /settings
- Expand settings: git provider selector (Gitea / GitHub / GitLab) with per-provider fields
- Add lib/git.ts: unified pushIncidentToGit() for Gitea, GitHub (PUT), and GitLab APIs
- Update incident detail screen to use lib/git.ts and dynamic provider routing

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

app/_layout.tsx

@@ -1,4 +1,5 @@
 import { useEffect } from "react";
+import { Pressable, Text } from "react-native";
 import { Stack, router } from "expo-router";
 import { StatusBar } from "expo-status-bar";
 import { GestureHandlerRootView } from "react-native-gesture-handler";
@@ -28,7 +29,20 @@ export default function RootLayout() {
           animation: "slide_from_right",
         }}
       >
-        <Stack.Screen name="index" options={{ title: "SheetHappens" }} />
+        <Stack.Screen
+          name="index"
+          options={{
+            title: "SheetHappens",
+            headerRight: () => (
+              <Pressable
+                onPress={() => router.push("/settings")}
+                style={{ paddingLeft: 12, paddingRight: 4, paddingVertical: 6 }}
+              >
+                <Text style={{ color: Colors.primary, fontSize: 22, lineHeight: 26 }}>⚙</Text>
+              </Pressable>
+            ),
+          }}
+        />
         <Stack.Screen name="new" options={{ title: "New Incident", presentation: "modal" }} />
         <Stack.Screen name="incident/[id]" options={{ title: "Incident" }} />
         <Stack.Screen name="settings" options={{ title: "Settings" }} />