fix(usage): keep significant decimals in cost tooltip rate multiplier

formatMultiplier rounded any value >= 0.01 with toFixed(2), so a
configured multiplier like 0.035 displayed as 0.04x in the /usage and
/admin/usage cost tooltips. Format with up to 4 decimals and trim
trailing zeros while keeping at least 2 decimals; display-only fix,
billing amounts were already computed from the true multiplier.

frontend/src/utils/__tests__/formatMultiplier.spec.ts

@@ -0,0 +1,27 @@
+import { describe, expect, it } from 'vitest'
+import { formatMultiplier } from '../formatters'
+
+describe('formatMultiplier', () => {
+  it('keeps significant decimals instead of rounding to 2 places', () => {
+    expect(formatMultiplier(0.035)).toBe('0.035')
+    expect(formatMultiplier(0.125)).toBe('0.125')
+    expect(formatMultiplier(0.015)).toBe('0.015')
+    expect(formatMultiplier(0.0625)).toBe('0.0625')
+  })
+
+  it('pads to at least 2 decimals for round values', () => {
+    expect(formatMultiplier(0.3)).toBe('0.30')
+    expect(formatMultiplier(1)).toBe('1.00')
+    expect(formatMultiplier(1.5)).toBe('1.50')
+    expect(formatMultiplier(2)).toBe('2.00')
+  })
+
+  it('handles small values down to 4 decimals', () => {
+    expect(formatMultiplier(0.001)).toBe('0.001')
+    expect(formatMultiplier(0.0001)).toBe('0.0001')
+  })
+
+  it('falls back to 2 significant digits below 0.0001', () => {
+    expect(formatMultiplier(0.00005)).toBe('0.000050')
+  })
+})

frontend/src/utils/formatters.ts

@@ -8,11 +8,10 @@ export function formatCacheTokens(tokens: number): string {
 }
 
 /**
- * 自适应精度格式化倍率（确保小数值如 0.001 不被截断）
+ * 自适应精度格式化倍率：保留至多 4 位小数并去掉末尾多余的 0，
+ * 但至少保留 2 位小数（0.035 -> "0.035"，0.3 -> "0.30"，1 -> "1.00"）
  */
 export function formatMultiplier(val: number): string {
-  if (val >= 0.01) return val.toFixed(2)
-  if (val >= 0.001) return val.toFixed(3)
-  if (val >= 0.0001) return val.toFixed(4)
-  return val.toPrecision(2)
+  if (val < 0.0001) return val.toPrecision(2)
+  return val.toFixed(4).replace(/(\.\d{2}\d*?)0+$/, '$1')
 }