fix: derive active worktree from sidebar cursor

[sebasv]

Summary

Adding or removing a worktree updated the sidebar selection but left the main pane showing the previously-active worktree (or the placeholder even when the cursor had moved to a survivor). Sidebar selection (ui.cursor, index-based) and main-pane driver (ui.active_worktree, path-based) were two independent fields that every mutation site had to remember to sync — and several didn't.

This makes the cursor the single source of truth: active_worktree_id() derives from ui.cursor, so navigating in the sidebar always refreshes the main pane. No separate "commit" step needed.

• remap_worktree_state also remaps the cursor by path, with fallbacks for vanished paths and emptied lists, so reorders and FS events keep the main pane in sync.
• try_create_worktree_modal lands the cursor on the just-created worktree by path lookup — git lists alphabetically, so the previous "tail of the list" assumption picked the wrong row on macOS.
• Persistence still uses path identity for cross-session survival: to_persisted derives the path from the cursor; apply_persisted resolves it back to indices.

Test plan

• cargo test — 142/142 pass, including new tests for cursor remap on reorder, on missing-path fallback, and on emptied-list fallback
• cargo clippy --all-targets -- -D warnings clean
• cargo fmt --check clean
• Manual: add a worktree → cursor lands on it, main pane shows its content immediately
• Manual: delete a worktree → cursor lands on a sibling, main pane shows the sibling's content
• Manual: delete the last worktree of a repo → cursor jumps to the parent repo, main pane shows placeholder
• Manual: arrow-navigate through sidebar → main pane follows in lockstep
• Manual: persisted active selection survives a restart when the path still exists; gracefully drops when it doesn't

🤖 Generated with Claude Code