Add background switcher across all themes

[TonciZ]

Summary

• Adds omarchy-theme-bg-switcher-all (omarchy theme bg-switcher-all), which opens the existing generic image picker over every theme's backgrounds/ folder at once (user themes, stock themes, and the per-theme user-override folder), instead of just the current theme.
• New menu entry style.background-all ("All Backgrounds"), same apply path as the existing background switcher (omarchy-theme-bg-set).
• New default bind SUPER+CTRL+ALT+SPACE next to the existing SUPER+CTRL+SPACE (current-theme-only) switcher.

No changes needed to omarchy-menu-images or omarchy-theme-bg-set — both already generic enough to support this. Theme discovery is passive: any theme with a backgrounds/ folder under either theme root is picked up automatically, no registration step.

Test plan

• ./test/all — no new failures (4 pre-existing test/shell failures on master/this branch alike, unrelated: missing local omarchy-pkgs checkout, locate/snapper/unowned-system-paths suites)
• bash -n bin/omarchy-theme-bg-switcher-all
• Verified end-to-end on a live Omarchy install (ported this from a user-space version first): opens with backgrounds from multiple themes shown together, selecting one applies it and updates omarchy theme bg current
• hyprctl configerrors clean after adding the bind