Fix remote worktree diff compatibility

[jln13x]

Summary

• keep rendering the last successful status and diff snapshots when remote refreshes fail
• fall back to legacy combined worktree diffs for older environments
• advertise and check a worktree source-control capability before exposing mutation controls
• refresh both status and preview state and document the fork-specific compatibility behavior

Validation

• vp check (zero errors; existing unrelated warnings remain)
• vp run typecheck
• vp test apps/web/src/components/WorktreeSourceControl.logic.test.ts apps/server/src/environment/ServerEnvironment.test.ts
• live browser verification of the worktree file list and selected-file diff