feat(ui): add keyboard-driven multi-pane layouts

[kallupx]

Summary

• Add one to four independently navigable browser panes with side-by-side, stacked, nested, and 2×2 layouts.
• Add Ctrl+S pane-management chords and a compact header layout menu.
• Route navigation, search, previews, appearance, settings, and file operations through the active pane.
• Support cross-pane drag-and-drop, shared cut state, and operation invalidation.
• Add active-pane highlighting, clearer dividers, drag-safe previews, and responsive preview sizing.

Behavior

• Strata still starts with one pane; layouts are not persisted.
• New panes clone the active location while keeping independent history and selection.
• Side-by-side and stacked layouts can be subdivided up to four panes.
• Switching from 2×2 directly to Side by side or Stacked keeps the active row or column.
• Closing the last pane remains a no-op.
• The shared preview follows the active pane and opens at a responsive width.
• Dragging files between panes no longer triggers an accidental preview or launch.

mouse.mp4

keys.mp4

Keyboard controls

• Ctrl+S, V — split side by side
• Ctrl+S, S — split top and bottom
• Ctrl+S, H/J/K/L or arrows — focus a pane
• Ctrl+S, W — cycle panes
• Ctrl+S, C — close the active pane

Testing

• cargo fmt --all --check
• cargo check --all-targets --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-targets --all-features — 234 passed
• Manual smoke testing of pane layouts, keyboard navigation, previews, and cross-pane drag-and-drop

Closes #47

[kallupx]

fixed the pane divider lines in the latest commit, so they are now thinner than in the videos.

[l0gicgate]

You'll have to rebase on main. Had to rewrite git history to remove Claude attributions

[l0gicgate]

This is a good feature we should think of: