refactor: extract formatCompactNumber util and add last_used_at to refresh key

- Add formatCompactNumber() for consistent large-number formatting (K/M/B)
- Include last_used_at in OpenAI usage refresh key for better change detection
- Add .gitattributes eol=lf rules for frontend source files

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

@@ -8,6 +8,7 @@ describe('buildOpenAIUsageRefreshKey', () => {
       platform: 'openai',
       type: 'oauth',
       updated_at: '2026-03-07T10:00:00Z',
+      last_used_at: '2026-03-07T09:59:00Z',
       extra: {
         codex_usage_updated_at: '2026-03-07T10:00:00Z',
         codex_5h_used_percent: 0,
@@ -27,12 +28,35 @@ describe('buildOpenAIUsageRefreshKey', () => {
     expect(buildOpenAIUsageRefreshKey(base)).not.toBe(buildOpenAIUsageRefreshKey(next))
   })
 
+  it('会在 last_used_at 变化时生成不同 key', () => {
+    const base = {
+      id: 3,
+      platform: 'openai',
+      type: 'oauth',
+      updated_at: '2026-03-07T10:00:00Z',
+      last_used_at: '2026-03-07T10:00:00Z',
+      extra: {
+        codex_usage_updated_at: '2026-03-07T10:00:00Z',
+        codex_5h_used_percent: 12,
+        codex_7d_used_percent: 24
+      }
+    } as any
+
+    const next = {
+      ...base,
+      last_used_at: '2026-03-07T10:02:00Z'
+    }
+
+    expect(buildOpenAIUsageRefreshKey(base)).not.toBe(buildOpenAIUsageRefreshKey(next))
+  })
+
   it('非 OpenAI OAuth 账号返回空 key', () => {
     expect(buildOpenAIUsageRefreshKey({
       id: 2,
       platform: 'anthropic',
       type: 'oauth',
       updated_at: '2026-03-07T10:00:00Z',
+      last_used_at: '2026-03-07T10:00:00Z',
       extra: {}
     } as any)).toBe('')
   })

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

@@ -0,0 +1,22 @@
+import { describe, expect, it } from 'vitest'
+import { formatCompactNumber } from '../format'
+
+describe('formatCompactNumber', () => {
+  it('formats boundary values with K/M/B', () => {
+    expect(formatCompactNumber(0)).toBe('0')
+    expect(formatCompactNumber(999)).toBe('999')
+    expect(formatCompactNumber(1000)).toBe('1.0K')
+    expect(formatCompactNumber(999999)).toBe('1000.0K')
+    expect(formatCompactNumber(1000000)).toBe('1.0M')
+    expect(formatCompactNumber(1000000000)).toBe('1.0B')
+  })
+
+  it('supports disabling billion unit (requests style)', () => {
+    expect(formatCompactNumber(1000000000, { allowBillions: false })).toBe('1000.0M')
+  })
+
+  it('returns 0 for nullish input', () => {
+    expect(formatCompactNumber(null)).toBe('0')
+    expect(formatCompactNumber(undefined)).toBe('0')
+  })
+})