Add battle-map controls for size, object density, and terrain mix

[Daren9m]

Problem

Encounter maps are generated with fixed defaults and no encounter-page controls for size, object density, or terrain variation.

Acceptance criteria

• DMs can choose map width and height before regeneration.
• DMs can control object/feature density.
• DMs can adjust terrain variation appropriate to the selected environment.
• Controls work from both the encounter page and standalone map generator.
• Map options persist locally and seeded generation stays deterministic for the same options.
• Generator tests cover size bounds, low/high density, and terrain variation.