Add option to span background image across window

[damyan94]

Summary

• Add a per-profile option to span a background image across the window.
• Scale and align the image using the top-level window dimensions.
• Have each terminal paint the corresponding window-relative section.
• Preserve the existing per-terminal behavior by default.
• Reuse the existing drawing modes, alignment, and shading settings.

Motivation

Closes #608.

This is an alternative approach to the draft implementation in #612.
Instead of introducing a window-owned background image, it preserves the
existing per-profile image configuration and aligns each terminal's rendering
in window coordinates. This avoids choosing which terminal profile owns the
window background and does not alter the Window/Notebook/Paned hierarchy.

Related to #635. Builds on the drawing modes introduced by #713.

Verification

• Python syntax parsing passed.
• Glade XML parsing passed.
• Manually tested with multiple panes, resizing, and fullscreen.
• Full automated test suite: not run.

AI assistance

Codex assisted with codebase inspection and implementation.