Add an About section to the account menu (#51)

Shows the app version, links the AGPL-3.0-or-later license text, and
links the source repo -- AGPL's own suggested-usage text recommends
exactly this ("if your software can interact with users remotely...
its interface could display a 'Source' link"), not just a courtesy
credits screen.

Version comes from package.json at build time via a Vite `define`
(__APP_VERSION__), so it can't drift from what's actually released.
License text is served at /LICENSE via a frontend/public/ symlink to
the repo-root LICENSE, the same pattern already used for the user
guide. Also bumps both package manifests to 1.0.0 ahead of tagging
the first release.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/vite.config.ts

@@ -1,9 +1,18 @@
+import { readFileSync } from 'node:fs'
 import { defineConfig } from 'vite'
 import react from '@vitejs/plugin-react'
 import { VitePWA } from 'vite-plugin-pwa'
 
+// Single source of truth for the version shown in the app's About dialog --
+// read at build time so it can never drift from what's actually released,
+// rather than a separately-maintained string in the component itself.
+const { version } = JSON.parse(readFileSync(new URL('./package.json', import.meta.url), 'utf-8'))
+
 // https://vite.dev/config/
 export default defineConfig({
+  define: {
+    __APP_VERSION__: JSON.stringify(version),
+  },
   plugins: [
     react(),
     VitePWA({