feat(worktrees): configurable base branch + local/remote picker

[im-ian]

Summary

Project worktrees can now start from a branch selected per project while preserving Acorn's existing automatic default behavior.

1. Project setting — Added a searchable worktree base-branch selector with Automatic, detected local branches, detected remote-tracking branches, and direct branch-name input.
2. Repository-backed choices — Added a backend branch-list command that reads the local repository's live refs and omits symbolic remote HEAD entries.
3. Creation behavior — Applied the configured branch to isolated sessions, prepared chat worktrees, and IPC-created sessions, with save-time validation for stale or missing refs.
4. Compatibility and coverage — Kept existing settings files backward-compatible, retained the main/master/HEAD fallback when Automatic is selected, and covered persistence, resolution, listing, and UI flows.

Expected impact

Area	Before	After
New Acorn worktrees	Always used the automatic main/master/HEAD fallback	Use the project's configured base branch, or the same fallback when Automatic is selected
Project settings UX	No worktree base-branch control	Searchable local/remote selector plus custom input
Invalid configured refs	Could not be configured	Rejected while saving before they can affect worktree creation

Design notes

• Detected selections are persisted as full refs (refs/heads/... or refs/remotes/...) so identically named local and remote branches remain unambiguous.
• Custom shorthand remains supported and resolves local branches first, then named remote-tracking refs, then origin/<name>.
• Branch discovery reads refs already present in the repository and does not perform a network fetch.
• Automatic remains represented by a null setting, preserving existing project-settings serialization and worktree fallback behavior.

Test plan

• pnpm run typecheck
• pnpm run test — 109 files, 1,289 tests passed
• pnpm exec playwright test tests/e2e/project-settings.spec.ts — 9 tests passed
• pnpm run build
• cargo test --manifest-path src-tauri/Cargo.toml project_settings::tests --lib — 5 tests passed
• cargo test --manifest-path src-tauri/Cargo.toml worktree::tests --lib — 28 tests passed
• cargo check --manifest-path src-tauri/Cargo.toml
• Targeted rustfmt --check for the modified Rust modules
• git diff --check

Notes

• Full-repository cargo fmt --check still reports the pre-existing formatting difference in src-tauri/src/pty_env.rs; this PR does not modify that file.
• cargo check reports five pre-existing dead-code warnings.