Add Layout Helper and shared window dividers

[kiteretsu903]

1. Layout Helper: I added a chooser to fill the remaining space after snapping. Escape or a click outside dismisses it. It follows normal snapping, so size-limited windows keep their actual size and use the existing alignment setting.

2. Shared dividers: I added dragging to resize adjacent left/right or top/bottom windows together. Double-click restores an equal split.

3. Defaults and permissions: Both features are off by default. Thumbnails require Screen Recording access on macOS 14 or later. Without it, the chooser still shows icons and titles. Enhanced divider transitions also use Screen Recording.

4. Keyboard and preview fixes: When the helper opens without keyboard input, the first Tab or arrow key selects the first available window. Keyboard-triggered opening already selects the first window, so navigation advances normally. I also fixed the seams between preview images and card headers, and kept the keyboard selection outline visible.

Depends on #1888 and #1889. The diff currently includes prerequisite changes.

[kiteretsu903]

Closing in favor of the focused replacements: #1906, #1907.

The changes have been reorganized to keep each feature easier to review and limit changes to shared infrastructure. Prerequisite-dependent replacements are drafts and explain which earlier changes are included. The original discussion remains here for reference.