fix: token tracking pipeline + premium usage dashboard redesign

Backend fixes:
- Fix totalTokens defaulting to 0 when API omits totalTokenCount (now computes prompt+completion)
- Fix streaming metadata extraction: unwrap response wrapper in Anthropic/Gemini handlers
- Fix Zod schemas: handle SQL SUM(null) with nullable().transform()
- Add is_estimated column migration to token_usage table

Frontend redesign:
- Rewrite Usage page with area charts (nivo/line), horizontal bar model ranking
- StatCard: add trend indicators (% vs previous period) and inline SVG sparklines
- Premium color palette, prompt/completion ratio bar, top models mini-list
- Live indicator with 15s auto-refresh

Design system migration:
- Standardize all components on Gemini Nexus tokens (rounded-xl, border-white/[0.06])
- Refine UI components, proxy tabs, cloud account list, provider groups

src/lib/nivo-theme.ts

@@ -1,61 +1,65 @@
-import type { PartialTheme } from '@nivo/theming';
-
-export const nivoTheme: PartialTheme = {
-  background: 'transparent',
-  text: {
-    fontFamily: "Geist, 'Microsoft YaHei', 'Heiti SC', 'WenQuanYi Micro Hei', sans-serif",
-    fontSize: 11,
-    fill: '#b8b8b8',
-  },
-  axis: {
-    domain: {
-      line: {
-        stroke: 'rgba(255,255,255,0.1)',
-        strokeWidth: 1,
-      },
-    },
-    ticks: {
-      line: {
-        stroke: 'transparent',
-        strokeWidth: 0,
-      },
-      text: {
-        fontFamily: "Geist, 'Microsoft YaHei', 'Heiti SC', 'WenQuanYi Micro Hei', sans-serif",
-        fontSize: 11,
-        fill: '#b8b8b8',
-      },
-    },
-    legend: {
-      text: {
-        fontFamily: "Geist, 'Microsoft YaHei', 'Heiti SC', 'WenQuanYi Micro Hei', sans-serif",
-        fontSize: 12,
-        fill: '#b8b8b8',
-      },
-    },
-  },
-  grid: {
-    line: {
-      stroke: 'rgba(255,255,255,0.06)',
-      strokeWidth: 1,
-      strokeDasharray: '4 4',
-    },
-  },
-  legends: {
-    text: {
-      fontFamily: "Geist, 'Microsoft YaHei', 'Heiti SC', 'WenQuanYi Micro Hei', sans-serif",
-      fontSize: 12,
-      fill: '#b8b8b8',
-    },
-  },
-  tooltip: {
-    container: {
-      background: '#2e2e2e',
-      color: '#fff',
-      fontFamily: "Geist, 'Microsoft YaHei', 'Heiti SC', 'WenQuanYi Micro Hei', sans-serif",
-      fontSize: 12,
-      borderRadius: 6,
-      boxShadow: '0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06)',
-      padding: '8px 12px',
-    },
-  },
-};
+import type { PartialTheme } from '@nivo/theming';
+
+/**
+ * Nivo chart theme aligned with Gemini Nexus design system.
+ * Uses Inter font family and refined dark palette.
+ */
+export const nivoTheme: PartialTheme = {
+  background: 'transparent',
+  text: {
+    fontFamily: "Inter, -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif",
+    fontSize: 11,
+    fill: '#6b7280',
+  },
+  axis: {
+    domain: {
+      line: {
+        stroke: 'rgba(255,255,255,0.06)',
+        strokeWidth: 1,
+      },
+    },
+    ticks: {
+      line: {
+        stroke: 'transparent',
+        strokeWidth: 0,
+      },
+      text: {
+        fontFamily: "'JetBrains Mono', monospace",
+        fontSize: 10,
+        fill: '#6b7280',
+      },
+    },
+    legend: {
+      text: {
+        fontFamily: "Inter, sans-serif",
+        fontSize: 11,
+        fill: '#6b7280',
+      },
+    },
+  },
+  grid: {
+    line: {
+      stroke: 'rgba(255,255,255,0.04)',
+      strokeWidth: 1,
+    },
+  },
+  legends: {
+    text: {
+      fontFamily: "Inter, sans-serif",
+      fontSize: 11,
+      fill: '#6b7280',
+    },
+  },
+  tooltip: {
+    container: {
+      background: '#16161e',
+      color: '#e8eaed',
+      fontFamily: "Inter, sans-serif",
+      fontSize: 12,
+      borderRadius: 8,
+      border: '1px solid rgba(255,255,255,0.06)',
+      boxShadow: '0 8px 24px -4px rgba(0, 0, 0, 0.3)',
+      padding: '8px 12px',
+    },
+  },
+};