fix: reset browser UA button chrome for dark mode

HTML buttons get a ~#efefef background and 2px outset border from the
browser UA stylesheet. In light mode these blend in; in dark mode they
render as stark white boxes. Adding a global button reset in theme.css
clears the UA defaults — explicit bg-* utility classes still win.

Affects: theme toggle, hamburger nav button, dashboard diagnose buttons,
and all other icon/text buttons that had no explicit bg class.

Bumps version to 0.6.2.

app/rest.py

@@ -187,7 +187,7 @@ async def _lifespan(app: FastAPI):
                 pass
 
 
-app = FastAPI(title="Turnstone API", version="0.6.1", docs_url="/turnstone/docs", redoc_url=None, lifespan=_lifespan)
+app = FastAPI(title="Turnstone API", version="0.6.2", docs_url="/turnstone/docs", redoc_url=None, lifespan=_lifespan)
 
 app.add_middleware(
     CORSMiddleware,

web/src/style/theme.css

@@ -46,6 +46,13 @@
   --badge-critical-bg:  #450a0a; --badge-critical-text: #f87171;
 }
 
+/* Reset UA button chrome — utility classes with higher specificity will override */
+button {
+  background: transparent;
+  border: 0;
+  cursor: pointer;
+}
+
 /* Smooth theme transitions */
 @media (prefers-reduced-motion: no-preference) {
   *, *::before, *::after {