Web frontend: hide app title and row titles, regroup help sheet, drop Ctrl+d/u

- New SHOW_APP_TITLE switch (off): the mascot + title header goes, the status pill
  stays on a slim row of its own.
- SHOW_ROW_TITLES off; with only icons, the shelf icon moves into a column left of
  the tiles so an icon-only shelf is no taller than one without.
- Help sheet regrouped into sections in a compact two-column grid, and reachable
  from a small keyboard button (not on phones or the typing page).
- Ctrl+d/Ctrl+u half-page jumps removed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/App.tsx

@@ -663,6 +663,31 @@ export function App() {
       )}
       {PERF_ENABLED && perfOpen && <PerfPanel onClose={() => setPerfOpen(false)} />}
 
+      {/* The F1 sheet, reachable without knowing about F1. Not on a phone (no keyboard)
+          or the typing page (TippenApp has its own help and owns every key there). */}
+      {!phone && ui.page !== "typing" && !ui.showHelp && (
+        <button
+          onClick={() => setUi((previous) => ({ ...previous, showHelp: true }))}
+          aria-label="Tastenkürzel"
+          style={{
+            position: "absolute",
+            left: PERF_ENABLED && !perfOpen ? 48 : 8,
+            bottom: 8,
+            zIndex: 5,
+            width: 32,
+            height: 32,
+            border: "none",
+            borderRadius: 999,
+            cursor: "pointer",
+            background: "oklch(15% 0.03 210 / .35)",
+            color: "#fff",
+            opacity: 0.5,
+          }}
+        >
+          ⌨
+        </button>
+      )}
+
       {(!state || library.loading) && <Splash error={library.error} />}
 
       {debugDynamicUI && debugSnapshot && (