CMM-952: Fix invisible back arrow on white images in Reader photo viewer

[nbradbury]

Description

In the Reader's full-screen photo viewer, the toolbar's back arrow is always rendered white (via ThemeOverlay.AppCompat.Dark.ActionBar) on a fully transparent toolbar. When the viewed image is white or very light, the arrow blends into it and becomes invisible.

This adds a dark gradient scrim behind the toolbar (reader_photo_viewer_toolbar_scrim.xml, set as the toolbar's background) so the white arrow always has contrast, regardless of image color.

Testing instructions

Reader photo viewer contrast:

1. Open a Reader post that contains a white or very light image (one is attached below).
2. Tap the image to open the full-screen photo viewer.

• Verify the back arrow is clearly visible against the light image.

3. Tap the screen to toggle the toolbar.

• Verify the scrim fades in/out together with the arrow, with no leftover dark band when the toolbar auto-hides.

4. Repeat over a dark image and in dark mode.

• Verify the arrow remains visible and the scrim is not distracting.

5. In a post with multiple images, swipe between pages.

• Verify the scrim behaves consistently on every image.

[dangermattic]

	1 Warning
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[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Android by scanning the QR code below to install the corresponding build.

	App Name	WordPress Android
	Build Type	Debug
	Version	pr23126-1ab7cec
	Build Number	1498
	Application ID	org.wordpress.android.prealpha
	Commit	1ab7cec
	Installation URL	2st2h8cho891o

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Android by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Android
	Build Type	Debug
	Version	pr23126-1ab7cec
	Build Number	1498
	Application ID	com.jetpack.android.prealpha
	Commit	1ab7cec
	Installation URL	0g81qns203u88

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[adalpari]

Nice!