Show the average line for single-effort-type workouts too; rebrand header to geniusrun

Previously the dashed average pace/HR line only drew for warm-up/cool-down
laps. Workouts with only one kind of effort throughout (no warm-up/cool-down
segmentation at all, e.g. a plain Long Run) had nothing to compare the
actual trace against, so they now get the same dashed line spanning the
whole activity at its overall (duration-weighted) average.

UI-only rename: "smartrun" -> "geniusrun", prefixed with the genie emoji.

frontend/src/App.tsx

@@ -24,7 +24,7 @@ function App() {
   return (
     <div className="app">
       <header className="app-header">
-        <h1>smartrun</h1>
+        <h1>🧞‍♀️ geniusrun</h1>
         <nav className="tabs">
           {TABS.map((t) => (
             <button