video gen: make display-sleep opt-in and settable per render

[atomantic]

Summary

• The GPU-watchdog display-sleep mitigation for local MLX video renders (mlx useAsyncAction's unmount guard is a no-op on React 19, and its test can't fail #3267) defaulted to ON and was settings-only — flip it to OFF by default, since a render is a short, attended action and sleeping the screen unasked reads as a crash.
• Add a visible checkbox on the Video Gen form (shown only for models the mitigation applies to) so a user who hits the watchdog crash can opt in for just that render, without changing the install-wide default.
• The existing Settings → Media Generation toggle still sets the install-wide default; the new per-render checkbox starts from that default and can override it either direction.
• Refactored the shared isDisplaySleepEnabled gate (server/services/displayPower.js) to take a defaultEnabled flag, so LoRA training keeps its unrelated default-ON behavior (unattended multi-hour runs) while video gen's new wrapper (server/services/videoGen/displayPower.js) uses default-OFF — one shared darwin/flag check, two callers.

Test plan

• server/services/displayPower.test.js, server/services/videoGen/displayPower.test.js, server/services/loraTraining/displayPower.test.js, server/routes/videoGen.test.js, server/services/videoGen/local.test.js — all pass.
• client/src/pages/VideoGen.displaySleep.test.jsx (new) — covers the checkbox's default-from-settings behavior, per-render opt-out/opt-in, and that it's hidden for a runtime the mitigation doesn't apply to.
• Full server suite (1949 files / 39275 tests) and full client suite (841 files / 10329 tests) pass.