feat: show build SHA + timestamp in a small footer for deploy verification

Adds a BuildStamp component pinned to the bottom-right of every page
(desktop only, dim monospace text) so it's obvious at a glance which
build is currently running.

The git short SHA and an ISO build timestamp are injected at build time
via Vite's `define`, falling back to 'unknown' if git is not available.
ESLint config declares the two compile-time constants as readonly
globals so no per-file disable comments are needed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

vite.config.js

@@ -1,9 +1,24 @@
 import { defineConfig } from 'vite'
+import { execSync } from 'node:child_process'
 import react from '@vitejs/plugin-react'
 import tailwindcss from '@tailwindcss/vite'
 
+function getBuildSha() {
+  try {
+    return execSync('git rev-parse --short HEAD', { stdio: ['ignore', 'pipe', 'ignore'] })
+      .toString()
+      .trim()
+  } catch {
+    return 'unknown'
+  }
+}
+
 // https://vite.dev/config/
 export default defineConfig({
+  define: {
+    __BUILD_SHA__: JSON.stringify(getBuildSha()),
+    __BUILD_TIME__: JSON.stringify(new Date().toISOString()),
+  },
   plugins: [react(), tailwindcss()],
   server: {
     proxy: {