add git branch chooser

[hfellerhoff]

adds a branch chooser to desktop and mobile, backed by three new tauri git commands (current_branch, list_branches, checkout_branch).

• desktop: git-branch button in the preview header opens a searchable popover; pick a branch or create one from typed text (slugified, e.g. My Feature → my-feature). switching restarts the preview.
• mobile: "branch" row in site settings opens a searchable bottom-sheet drawer.
• lists local and remote branches in one deduped list (laptop/cloud icons); remote list refreshes best-effort in the background, works offline.
• safe checkout by default: if local edits would be lost, a dialog lists the at-risk files with cancel / destructive "discard changes and switch".
• remote-only branches materialize as local tracking branches on switch; publish sets upstream on first push.
• deployment status now follows the active branch instead of hardcoded main.
• hidden entirely when not in a git repo or on a detached head.

tested with cargo tests (incl. new real-libgit2 create/switch/conflict test), new useBranches vitest suite, typecheck, vp check, and playwright smoke runs against the browser mock on both platforms.