fix(panels): compile out the git button on web (no git backend)

The web/wasm build has no git backend and never paints GitPanel, so
the top-bar git button toggled an invisible panel (Codex stop-time
review). Gate the button's paint + hit-test on a `GIT_BUTTON_AVAILABLE`
const (`!cfg!(target_arch = "wasm32")`) so it only exists on desktop.

crates/op-editor-ui/src/widgets/top_bar.rs

@@ -32,6 +32,11 @@ const PAD: f32 = 12.0;
 const DIVIDER_W: f32 = 1.0;
 const DIVIDER_H: f32 = 14.0;
 const DIVIDER_GAP: f32 = 4.0;
+/// The git button needs a desktop git backend (`op-git` via the
+/// desktop host) to populate + paint the panel; the web/wasm build
+/// has none, so the button is compiled out there — otherwise it would
+/// toggle an invisible panel (Codex stop-time review).
+const GIT_BUTTON_AVAILABLE: bool = !cfg!(target_arch = "wasm32");
 /// Gap between the stacked per-agent brand icons in the chip.
 const AGENT_ICON_GAP: f32 = 4.0;
 /// Diameter of a macOS-style window-control dot.
@@ -370,8 +375,9 @@ impl TopBar {
         if rect_contains(figma_rect, point) {
             return Some(TopBarHit::OpenFigmaImport);
         }
-        // Git-panel toggle, just right of the centred file name.
-        if rect_contains(self.git_button_rect(rect), point) {
+        // Git-panel toggle, just right of the centred file name
+        // (desktop only — see `GIT_BUTTON_AVAILABLE`).
+        if GIT_BUTTON_AVAILABLE && rect_contains(self.git_button_rect(rect), point) {
             return Some(TopBarHit::ToggleGitPanel);
         }
         // Right cluster: Maximize / Sun / Globe-with-chevron (right→left).
@@ -587,9 +593,11 @@ impl Widget for TopBar {
         );
 
         // Git-panel button just right of the file name (TS GitButton):
-        // a branch glyph + optional branch name. Always shown — a
-        // click toggles the git panel (which offers `init` when the
-        // doc isn't yet in a repo).
+        // a branch glyph + optional branch name. Always shown on
+        // desktop — a click toggles the git panel (which offers `init`
+        // when the doc isn't yet in a repo). Compiled out on web,
+        // which has no git backend to paint a panel.
+        if GIT_BUTTON_AVAILABLE {
             let git_rect = self.git_button_rect(rect);
             draw_icon(
                 cx.backend,
@@ -612,6 +620,7 @@ impl Widget for TopBar {
                     Point2D::new(git_rect.origin.x + ICON_SIZE + 6.0, center_y + 4.0),
                 );
             }
+        }
 
         // ── Right cluster ──────────────────────────────────────
         // Right → left: Maximize | Sun | Globe+Chevron. Globe is a