refactor(gui): hoist background color onto shared base builder

[StudentWeis]

Summary

Deduplicate the repeated .bg(self.main_panel_surface(...)) call by computing surface_bg once and applying it on the shared base builder.

Linked Issue

Closes #130

Changes

• Compute surface_bg at the top of Render::render and chain .bg(surface_bg) onto base.
• Remove 4 redundant per-arm .bg(...) calls (Settings, About, Help, ClipboardList) and the deactivated-state duplicate.

Testing

• scripts/precheck.sh passes locally
• New / updated tests cover the change — N/A (purely mechanical, existing tests pass)

Self-Check

• PR title follows Conventional Commits
• No hardcoded user-facing strings — i18n keys added to all locale files
• Errors defined with thiserror (no manual impl Display/Error)
• No unrelated changes mixed in