Prioritize active projects in dashboard

[flemzord]

Summary

• Sort dashboard projects so entries with active worktrees appear first while preserving configured order within groups
• Reapply the active-first ordering after reloads and session refreshes without losing selection by project name
• Add a timeout for git preview loading so the preview pane does not stay on Loading indefinitely

Tests

• cargo test --manifest-path zigzag/Cargo.toml -p zigzag-tui
• cargo check --manifest-path zigzag/Cargo.toml --all