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/AppFooter.tsx

@@ -0,0 +1,43 @@
+/**
+ * Tiny corner footer pinned to the bottom-right of the main column.
+ * Renders "Built with hubris • <year in roman numerals>", refreshed
+ * once per page load.
+ */
+
+const ROMAN_PAIRS: [number, string][] = [
+  [1000, 'M'],
+  [900, 'CM'],
+  [500, 'D'],
+  [400, 'CD'],
+  [100, 'C'],
+  [90, 'XC'],
+  [50, 'L'],
+  [40, 'XL'],
+  [10, 'X'],
+  [9, 'IX'],
+  [5, 'V'],
+  [4, 'IV'],
+  [1, 'I'],
+]
+
+function toRoman(n: number): string {
+  let result = ''
+  for (const [value, symbol] of ROMAN_PAIRS) {
+    while (n >= value) {
+      result += symbol
+      n -= value
+    }
+  }
+  return result
+}
+
+export function AppFooter() {
+  const year = new Date().getFullYear()
+  return (
+    <div className="pointer-events-none absolute bottom-2 right-3 z-10 text-[10px] text-text-faint">
+      <span className="pointer-events-auto">
+        Built with hubris • {toRoman(year)}
+      </span>
+    </div>
+  )
+}

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">