The top bar had no affordance to open the git panel (only a
keyboard/menu path). Add a TS-style git button just right of the file
name: a `GitBranch` glyph + the current branch name when in a repo.
Always shown (per request) — a click toggles the git panel, which
offers `init` when the doc isn't yet a repo.
- new `Icon::GitBranch` lucide glyph;
- `TopBar.git_branch` from `git_panel.branch`; `git_button_rect`
(CJK-aware width estimate so it clears a CJK file name) shared by
paint + hit-test; `TopBarHit::ToggleGitPanel`;
- native host mirrors `main.rs` toggle bookkeeping (per-frame refresh
does the scan); web host toggles `git_panel.open`.
89b74604ba