feat(ui): add month view with income entry and post table

Negative remaining balances are shown in a warning treatment; nothing
blocks going over budget.

Applies the Tasks 18-23 design contract: --paper/--ink/--rule/--accent/
--surplus/--overspend tokens and the three type-role font variables in
src/index.css, --font-money on every money figure, and the carry meter
(a 3px hard-stopped gradient beneath each PostTable row) encoding
carried-in surplus, allocation track, spend fill and overrun.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/index.css

@@ -1,5 +1,25 @@
 @import "../styles/globals.css";
 
+/*
+ * Design contract tokens (Tasks 18-23).
+ * See .superpowers/sdd/2026-09-01-budget-app/design-contract.md
+ *
+ * Deliberately unlayered (not inside @layer base) so it wins the cascade
+ * over the shadcn defaults imported above, which are also unlayered.
+ */
+:root {
+  --paper: #f2f4f6; /* cool statement-paper ground, NOT cream */
+  --ink: #16212b; /* ink navy, primary text */
+  --rule: #d8dee4; /* hairlines, table borders */
+  --accent: #0f6e63; /* muted teal: links, primary actions, spend fill */
+  --surplus: #2f6f4e; /* carried-forward surplus */
+  --overspend: #a6402b; /* ochre-red: negative balances only */
+
+  --font-display: ui-sans-serif, "Segoe UI Variable Display", system-ui, sans-serif;
+  --font-body: system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;
+  --font-money: ui-monospace, "SF Mono", "Cascadia Mono", "Roboto Mono", monospace;
+}
+
 @layer base {
   :root {
     @apply font-sans;
@@ -7,6 +27,34 @@
 
   body {
     @apply grid place-items-center min-w-[320px] min-h-screen relative m-0 bg-background text-foreground;
+    font-family: var(--font-body);
+  }
+
+  h1,
+  h2,
+  h3,
+  h4,
+  h5,
+  h6 {
+    font-family: var(--font-display);
+    font-weight: 650;
+    letter-spacing: -0.02em;
+  }
+}
+
+@layer utilities {
+  /* Applied to every money figure: mono face, tabular digits. */
+  .font-money {
+    font-family: var(--font-money);
+    font-variant-numeric: tabular-nums;
+  }
+
+  .text-overspend {
+    color: var(--overspend);
+  }
+
+  .text-surplus {
+    color: var(--surplus);
   }
 }