Expose UI scale in client options

Add a working interface scale setting to PRK.Client options using the existing logical-coordinate renderer and machine preference. Remove --ui-scale per user direction; saved preferences are the single source of truth. Reuse options draft/apply/discard patterns; persist the applied scale and support runtime relayout. Work in a separate worktree based on develop, leaving the original working copy untouched. Acceptance: users can change interface scale, apply and retain it across restarts; discard leaves the applied scale unchanged; relevant client tests and build pass. Verified: regression test failed with the unconnected scale control, then passed after wiring. Client suite: 4398 passed, 187 skipped; graphics suite: 721 passed, 1 skipped. Warnings-as-errors client build: zero warnings/errors. git diff --check clean. Interactive game-client visual testing was not performed. Pull request #2939: https://git.project-rk.com/prk/engines/pulls/2939