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.ts

@@ -0,0 +1,19 @@
+import { useState, useEffect } from 'react'
+import { invoke } from '@tauri-apps/api/core'
+import { isTauri, mockInvoke } from '../mock-tauri'
+
+function tauriCall<T>(cmd: string): Promise<T> {
+  return isTauri() ? invoke<T>(cmd) : mockInvoke<T>(cmd)
+}
+
+export function useBuildNumber(): string | undefined {
+  const [buildNumber, setBuildNumber] = useState<string>()
+
+  useEffect(() => {
+    tauriCall<string>('get_build_number').then(setBuildNumber).catch(() => {
+      setBuildNumber('b?')
+    })
+  }, [])
+
+  return buildNumber
+}