feat: Add About section with version info and update check (#50)

* feat: add About section with version info and update check

- Add About menu item in user dropdown
- Show frontend and backend versions separately
- Add 'Check for Updates' feature using GitHub API
- Compare versions using semver logic
- Cache update check results in sessionStorage (1 hour TTL)
- Link to GitHub repository
- Add i18n translations for EN and DE
- Extend health endpoint to return backend version

* fix: correct i18n interpolation in About modal

- Fix copyright year using dynamic interpolation
- Fix update available message (remove duplicate version placeholder)
- Add download link for available updates
- Change license to GPL-3.0

* fix: correct license to MIT

* chore: sync package.json versions to v1.3.1

frontend/src/i18n/de.json

@@ -420,5 +420,26 @@
     "save": "Korrektur speichern",
     "saving": "Speichern...",
     "success": "Bestand erfolgreich korrigiert"
+  },
+  "about": {
+    "title": "Über",
+    "appName": "MedAssist-ng",
+    "description": "Open-Source Medikamentenverwaltung und Planungsanwendung für selbst gehostete Umgebungen.",
+    "version": "Version",
+    "frontend": "Frontend",
+    "backend": "Backend",
+    "checkForUpdates": "Nach Updates suchen",
+    "checking": "Prüfe...",
+    "upToDate": "Du bist auf dem neuesten Stand!",
+    "updateAvailable": "Update verfügbar",
+    "viewOnGitHub": "Auf GitHub ansehen",
+    "downloadUpdate": "Update herunterladen",
+    "checkFailed": "Update-Prüfung fehlgeschlagen",
+    "lastChecked": "Zuletzt geprüft",
+    "github": "GitHub",
+    "license": "MIT-Lizenz",
+    "copyright": "© {{year}} Daniel Volz",
+    "madeWith": "Mit ❤️ erstellt für besseres Gesundheitsmanagement",
+    "techStack": "Entwickelt mit React, Fastify & SQLite"
   }
 }

frontend/src/i18n/en.json

@@ -422,5 +422,26 @@
     "save": "Save Correction",
     "saving": "Saving...",
     "success": "Stock corrected successfully"
+  },
+  "about": {
+    "title": "About",
+    "appName": "MedAssist-ng",
+    "description": "Open-source medication tracking and planning application for self-hosted environments.",
+    "version": "Version",
+    "frontend": "Frontend",
+    "backend": "Backend",
+    "checkForUpdates": "Check for Updates",
+    "checking": "Checking...",
+    "upToDate": "You're up to date!",
+    "updateAvailable": "Update available",
+    "viewOnGitHub": "View on GitHub",
+    "downloadUpdate": "Download Update",
+    "checkFailed": "Could not check for updates",
+    "lastChecked": "Last checked",
+    "github": "GitHub",
+    "license": "MIT License",
+    "copyright": "© {{year}} Daniel Volz",
+    "madeWith": "Made with ❤️ for better health management",
+    "techStack": "Built with React, Fastify & SQLite"
   }
 }