Feature: restrict the visible map to the conflict area when a Focus is set

[fank]

Feature request

Problem

The Focus feature currently trims the timeline to a time range (in/out frames), but the map still shows the entire world — including both teams' preparation and setup phases staged far from the actual engagement. During review this is noisy: you watch units maneuvering off-area while the fight you care about is happening elsewhere.

The idea: give a Focus an optional area (conflict zone) so that only the conflict area is visible while that focus is active.

Desired behavior

When a Focus with an area is active:

• The timeline is already trimmed to the focus range (existing behavior — this "cuts off preparation" in time and jumps you toward the action start).
• Units/events whose position at the current frame is outside the area are hidden — only the units involved in the conflict render. Both teams' off-area preparation disappears from the map.

Proposed UX

• A "Set area to current view" action, parallel to the existing "set in-point / out-point to playhead" controls — captures the current map viewport as the conflict area. This keeps it symmetrical with how Focus already works (no new drawing surface required).
• A way to clear the area back to showing everything.

Alternatives considered

• Full polygon-drawing editor + server-side action statistics — much more powerful, but a large scope and more than this playback-visibility need requires.
• Camera clamp only (auto-pan/lock the view, keep all units) — doesn't satisfy "preparation of both teams cut off", since off-area units would still be visible.

Implementation sketch

• Small pointInPolygon utility.
• Optional focusArea on a recording's focus metadata (parallel to focusStart/focusEnd).
• In the Leaflet renderer, hide any entity whose position at the current frame is outside the active area, reusing the existing per-frame visibility path already used by the name-display modes.