Recording: keep the display awake while a recording runs

[EtienneLescot]

Part of #920 (rank 28). Effort S, risk none.

Problem

• Nothing keeps the display or the system awake while a recording runs (no powerSaveBlocker anywhere in electron/ or src/).
• A long passive take (a webinar, a demo left running) can reach the display sleep timeout: the capture goes black, or the machine sleeps mid-take. Hypothesis, not observed yet.

Proposal

• powerSaveBlocker.start("prevent-display-sleep") when a recording starts, stop when it stops, pauses included, on every platform and backend.

Verify

• A unit test with Electron mocked: started once on record, stopped on stop and on every error path, never leaked.
• On a real machine: a 20 min passive take with a 5 min display timeout.

[github-actions]

Fixed by #939 and merged into main.

This change is assigned to the Next Release release milestone and is not necessarily available in the latest downloadable desktop release yet. It is currently marked as status: pending release until a packaged release containing it is published.

[EtienneLescot]

Moved into v2.0.0 as part of the pre-release cut.