fix(app): soften the mobile diff tree button and remove row shadows (#4898)

Use a neutral surface for diff navigation and restrict Android elevation to document headers so shared tree rows stay flat after scrolling.

packages/app/e2e/mobile/diff-tree/README.md

@@ -0,0 +1,32 @@
+# Mobile diff tree appearance
+
+## Reproduce the row shadow
+
+Use an Android emulator with a workspace containing 80 modified text files,
+named `file-000.txt` through `file-079.txt`. Each file changes `before` to `after`.
+
+1. Open the workspace's Changes sidebar, then **Jump to file**.
+2. Swipe upward through the tree until `file-079.txt` is visible.
+3. Inspect the row where the scroll gesture started. It must remain flat.
+4. Swipe back to the first file, then return to the last file and select it.
+   The sheet should close and show that file's diff.
+
+## Recorded verification
+
+Android API 35, 1080×1920. The baseline produced a raised gray shadow around
+`file-074.txt` after scrolling. The fixed build reached the same last row without
+the shadow. Baseline scrolling in both directions and selection of the last file
+passed; the fixed build also reached the last row and closed the sheet on selection.
+The fixed-build assertion for the selected diff header did not match, so it does
+not establish the final jump position.
+
+| Before                             | After                            |
+| ---------------------------------- | -------------------------------- |
+| ![Row shadow](evidence/before.png) | ![Flat rows](evidence/after.png) |
+
+| Light                             | Dark                            |
+| --------------------------------- | ------------------------------- |
+| ![Light diff](evidence/light.png) | ![Dark diff](evidence/dark.png) |
+
+The shadow check is visual; the existing header-interaction unit test does not
+test Android rendering. iOS, browser web, and Electron were not manually exercised.

packages/app/src/components/ui/floating-action-button.tsx

@@ -53,13 +53,13 @@ const styles = StyleSheet.create((theme) => ({
     alignItems: "center",
     justifyContent: "center",
     borderRadius: theme.borderRadius.full,
-    backgroundColor: theme.colors.accent,
+    backgroundColor: theme.colors.surface3,
     ...theme.shadow.md,
   },
   pressed: {
     backgroundColor: theme.colors.interactionHighlight,
   },
   glyph: {
-    color: theme.colors.accentForeground,
+    color: theme.colors.foreground,
   },
 }));

packages/app/src/git/file-header.tsx

@@ -376,10 +376,10 @@ const styles = StyleSheet.create((theme) => ({
     gap: theme.spacing[1],
     minWidth: 0,
     zIndex: 2,
-    elevation: 2,
     userSelect: "none",
   },
   documentHeader: {
+    elevation: 2,
     height: 28,
     paddingLeft: 0,
     paddingRight: 0,