feat: add environment browsing to encounter editor compendium sheet

[gwillish]

Context

PR #154 replaced CompendiumBrowserView with CompendiumSelectorSheet in the encounter editor. CompendiumSelectorSheet is adversary-only; environment browsing from the editor was not carried forward.

The setEnvironment(_:) method in EncounterBuilderView was removed in #154 as dead code. When this is implemented, that method will need to be re-added (or an equivalent path wired up).

Work

Add environment selection to the encounter editor. Options:

1. Extend CompendiumSelectorSheet with a mode/tab to switch between adversaries and environments.
2. Create a separate EnvironmentSelectorSheet and add a second "Browse Environments" button in the editor.

Either way, tapping an environment row should call setEnvironment(_:) (or equivalent) and dismiss the sheet.

Acceptance criteria

• GM can browse and add an environment from within the encounter editor
• Selecting an environment dismisses the sheet and sets it on the draft
• Replacing an existing environment works (single-environment constraint)