fix(css): an icon-only grid button centres its icon again

Obsidian styles every `button` with `justify-content: center`, so the reset
answers with `flex-start` - which also lands on a grid button and overrides
the inline half of its `place-items: center`. The rail's connection dot came
out pinned 12px left of centre inside its own hit area.

tests/styles.test.ts

@@ -89,4 +89,12 @@ describe("cascade against Obsidian", () => {
     expect(source).not.toMatch(/@import "tailwindcss"\s*[;l]/);
     expect(source).not.toMatch(/@import\s+"tailwindcss\/preflight/);
   });
+
+  // Obsidian's own `button` rules are what the reset answers, but a grid
+  // button must keep centring itself or an icon-only one drifts left.
+  it("leaves grid buttons to their own placement", () => {
+    expect(source).toMatch(
+      /button:is\(\.grid, \.inline-grid\) \{\s*justify-content: initial/
+    );
+  });
 });