feat: center hints over the timeline + add hubris footer

- KeyboardHints: switch from fixed positioning to absolute, mounted
  inside the main content column. The column is now relative-positioned
  so the hints overlay centers against the timeline area instead of the
  raw viewport (which was off-center because of the sidebars).
- AppFooter: tiny "Built with hubris • <YEAR in roman>" pinned to the
  bottom-right corner of the main column. Year is computed at render
  time and converted via a small toRoman helper.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/KeyboardHints.tsx

@@ -25,7 +25,11 @@ export function KeyboardHints() {
       ]
 
   return (
-    <div className="pointer-events-none fixed bottom-4 left-1/2 z-30 -translate-x-1/2">
+    // Absolute (not fixed) so the parent's flex/position context can
+    // center it relative to the timeline area, not the viewport. Mount
+    // inside the main column in App.tsx so it isn't offset by the
+    // sidebar widths.
+    <div className="pointer-events-none absolute bottom-4 left-1/2 z-30 -translate-x-1/2">
       <div className="pointer-events-auto flex items-center gap-3 rounded-full border border-border/60 bg-surface/40 px-4 py-1.5 shadow-lg ring-1 ring-white/5 backdrop-blur-md">
         {hints.map((hint, i) => (
           <div key={i} className="flex items-center gap-1.5">