feat: simplify About modal with single version link to GitHub release (#123)

- Replace separate Frontend/Backend versions with single app version
- Version is now a clickable link to the GitHub release page
- Replace stopwatch SVG with actual app logo (favicon.svg)
- Fix update check UX: previous result stays visible during re-check
- Add 1s minimum delay for update check spinner visibility
- Reserve space for update result to prevent modal jumping
- Remove unused i18n keys (frontend/backend)
- Update release-manager docs with version link info

frontend/src/i18n/de.json

@@ -474,8 +474,6 @@
     "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!",

frontend/src/i18n/en.json

@@ -474,8 +474,6 @@
     "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!",