feat(ui): ink & copper ledger retheme + analytics redesign

App-wide retheme done at the token layer: Tailwind's zinc scale is remapped
to warm ink/paper neutrals and indigo to copper in globals.css, so every
page inherits the palette. Fraunces (serif display) added for page titles
and hero figures; all figures now render in mono with tabular numerals.

Analytics page redesigned around a month spine — twelve clickable columns
scaled to each month's spend that act as hero, context, and period
navigation. Adds a 'What changed' top-movers panel vs the previous month,
replaces the 8-line category trend chart with per-category sparkline small
multiples, heat-tints the six-month ledger table, and restyles the Pareto,
pace chart, and cashflow strip. Kept: Pareto, cumulative-vs-typical pace,
drill-downs, regular/occasional split.

Insights and Merchants restyled to the same kit; chart tokens centralised
in category-colors.ts (CHART). Cleared the pre-existing lint errors in
insights (typed tooltip, removed any-casts).

src/app/shared/page.tsx

@@ -307,7 +307,7 @@ export default function SharedPage() {
   return (
     <div className="space-y-6">
       <div className="flex items-center justify-between gap-3">
-        <h2 className="text-xl font-semibold">Shared Expenses</h2>
+        <h2 className="text-2xl font-display">Shared Expenses</h2>
         <div className="flex items-center gap-2 ml-auto flex-wrap">
           <select
             value={participantId ?? ""}