fix(terrain-viewer): use Pointer Lock for drag pan and tilt

[kui]

Summary

• Drag-pan (left button) and drag-tilt (middle button) accumulate movementX/Y, but once the OS cursor reaches the window edge no more motion is delivered and the drag stalls.
• Acquire Pointer Lock on mousedown for the left or middle button, and release it on mouseup when neither button is held. A document-level mouseup listener guarantees cleanup if Pointer Lock is dropped externally (Escape key, focus loss).

Test plan

• deno task ci:fmt-check
• deno task ci:lint
• deno task ci:type-check
• deno task test
• Manual: drag-pan past the canvas edge in the terrain viewer and confirm camera keeps moving instead of stalling; verify the cursor reappears on release.