feat(app): add image backgrounds

[opencode-agent]

Summary

• add background image controls to appearance settings on web and desktop
• persist web images in Cache Storage and desktop images as bounded managed files
• serve desktop images through the restricted renderer protocol and synchronize changes across open windows
• make top-level app surfaces theme-aware and translucent while keeping controls readable

Checks

• bun typecheck (packages/app)
• bun typecheck (packages/desktop)
• bun test src/main src/renderer (packages/desktop) — 63 passed
• web picker, persistence across reload, and rendering verified in Chromium
• app unit suite — 604 passed; existing i18n parity failure remains for three unrelated session.header.reveal.* keys

Requested by: @Brendonovich (Brendan Allan via Slack)