Multi-display: add a capture mode (active / all / selected) — secondary screens are never captured

[ManmohanBuildsProducts]

Summary. On multi-monitor setups, capture only follows the active display (the one with the focused window / cursor) — one screen per tick, switching as you move. Content on secondary displays is never captured while you're working on another screen. For a "memory of what I was doing" tool this is a real recall gap, since the second monitor is often exactly where the context lives (a spec, dashboard, Slack, logs you're reading while working on the main screen). Passive reading on a non-focused screen never triggers a focus change, so it's invisible.

Current behavior (verified). The recorder enumerates screen.getAllDisplays() but captures a single targetDisplay / cursorDisplayId (falling back to primary). Captured stills are never simultaneous across displays — confirmed by frame timestamps (no two captures in the same second) and by the resolution of each frame tracking whichever display is active.

Why this isn't just a default to flip. Capturing all live screens has real costs:

• Storage/CPU scale ~N× with monitor count (the pipeline already downscales aggressively for cost).
• A static doc on a secondary screen re-captured every few seconds produces heavy near-duplicate frames (related: Perceptual frame deduplication to eliminate redundant I/O #10, perceptual frame dedup).
• Privacy: active-only means idle/secondary content stays uncaptured, which some users will want.

Proposal: make it configurable, default to active-only.

• active — current behavior (cheap, private).
• all — capture every live display each tick (full multi-monitor recall).
• selected — capture a chosen subset (e.g. monitors 1 and 2, exclude a TV/3rd screen).

This gives multi-monitor power users completeness without forcing the cost/privacy hit on everyone. Pairs naturally with the dedup work in #10 so an all mode doesn't drown in redundant frames.

Environment: macOS 26 (Tahoe), Familiar v0.0.70, Apple Silicon, dual-display.

[ManmohanBuildsProducts]

Related: #81 (Hindi/Devanagari OCR). Both are "completeness of capture" gaps — #81 is what text gets extracted from a frame, this is which screens get into frames at all. A user on a non-English, multi-monitor setup hits both at once.

[Hyaxia]

hey @ManmohanBuildsProducts, sure i think having the option to make familiar capture all screens is a good idea.
by default it should be just the screen where the mouse is on, but in the "advanced" area of the "Capture" tab we can have the option to capture all screens.