feat: refresh app icons and branding

[andre487]

Replace the old monitor/star branding with a gradient monitor/phone design: an open envelope on the computer and a closed envelope on the phone. Adaptive and round launcher icons include extra padding so the devices sit comfortably inside circular masks.

• Refresh launcher PNGs, adaptive layers, store icon, feature graphic and README artwork.
• Use a simplified white monitor/phone vector for notification and themed launcher icons.
• Provide self-contained SVGs with matching PNGs in assets/branding/ for web use and sharing, with export instructions.
• Remove obsolete notification PNGs, the old update drawable, unused background color, old banner master and prompt. Preserve the original artwork license.

Validation:

• ./gradlew :app:assembleDebug :app:lintDebug --offline --no-daemon passed.
• SVGs and local documentation links checked; store image dimensions and PNG transparency verified.
• Circular mask, notification icon at 24/18 px and refreshed artwork inspected in iTerm2.
• git diff --check passed.

Preview:

Round launcher / README	Store feature graphic

APK artifacts

• Download debug APK — signed with the disposable Android debug key
• Download unsigned release APK — verified unsigned

Built from commit bb0e8bfe by CI run #67.
Artifacts expire after 14 days. Neither APK uses the Message487 release key.