Add themed screenshots and useful troubleshooting guides

[SetZero]

Changes

• Add 36 theme-aware screenshot pairs covering desktop, mobile, and troubleshooting pages.
• Replace empty Notifications and Privacy captures with populated controls.
• Replace the illustrative wallpaper stub with an actual populated chat screenshot.
• Add a local live document canvas screenshot in both schemes.
• Rewrite all six troubleshooting guides with concrete comparisons, result interpretation, and relevant screenshots.

Verification

• Astro production build passes (62 pages).
• All referenced images exist; light/dark files differ.
• Browser checks verify theme selection and image loading for settings, personalization, live docs, mobile, and audio troubleshooting.

Capture scope

Screenshots render actual client components with local sample state. Mobile uses the Android browser shell. The document canvas is local and does not claim a deployed collaborative plugin session.