feat: show dynamic build number in status bar (bNNN format) (#163)

* feat: show dynamic build number in status bar (bNNN format)

Replace hardcoded v0.4.2 with a dynamic build number derived from
git rev-list --count HEAD at compile time. The count is embedded via
build.rs and exposed through a get_build_number Tauri command.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* design: add build-number-status-bar wireframes (status bar with dynamic b-number)

* fix: use runtime env var for BUILD_NUMBER (compile-time env! not available in CI)

* style: rustfmt format get_build_number

---------

Co-authored-by: Test <test@test.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

src/hooks/useBuildNumber.test.ts

@@ -0,0 +1,25 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { renderHook, waitFor } from '@testing-library/react'
+import { useBuildNumber } from './useBuildNumber'
+
+vi.mock('@tauri-apps/api/core', () => ({ invoke: vi.fn() }))
+vi.mock('../mock-tauri', () => ({
+  isTauri: () => false,
+  mockInvoke: vi.fn().mockResolvedValue('b223'),
+}))
+
+beforeEach(() => { vi.clearAllMocks() })
+
+describe('useBuildNumber', () => {
+  it('returns build number from mock invoke', async () => {
+    const { result } = renderHook(() => useBuildNumber())
+    await waitFor(() => expect(result.current).toBe('b223'))
+  })
+
+  it('returns fallback on error', async () => {
+    const { mockInvoke } = await import('../mock-tauri')
+    vi.mocked(mockInvoke).mockRejectedValueOnce(new Error('fail'))
+    const { result } = renderHook(() => useBuildNumber())
+    await waitFor(() => expect(result.current).toBe('b?'))
+  })
+})