feat(18-03): Task 2 — Einstellungsseite Desktop-App mit Knoepfen, Groesse und Erklaerung

- Neue Route /settings/general/desktop (DesktopSettingsPage)
- DesktopAppSettings: Version, zwei Primaerknoepfe mit Plattform-Symbol,
  Dateiname/Groesse je Knopf, Beta-Kanal-Hinweis, vier erklaerende Saetze,
  Hinweistext wenn keine Pakete hinterlegt sind
- Seitenleiste: Eintrag "Desktop-App" unter "Allgemein"
- de/en: settings.categoryDesktopApp, settings.desktop.*
- umlaut-dictionary.ts: "neuere" zur Allowlist ergaenzt (Rule 1 —
  Waechter-Test brach an einer bereits korrekten Umlautschreibung, die
  zufaellig die Buchstabenfolge "ue" enthaelt)

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

apps/web/src/messages/en.json

@@ -132,6 +132,7 @@
     "categoryCalendar": "Calendar",
     "categoryGeneral": "General",
     "categoryAccount": "Account",
+    "categoryDesktopApp": "Desktop App",
     "categorySmtp": "SMTP",
     "account": {
       "title": "Account",
@@ -152,6 +153,19 @@
       "deleteAvatarSuccess": "Profile picture deleted successfully.",
       "deleteAvatarError": "Failed to delete picture."
     },
+    "desktop": {
+      "title": "Desktop App",
+      "intro": "The desktop app opens Tessera in its own window – no browser, with an icon in the notification area of the taskbar.",
+      "firstStart": "On first start the app asks for the address of your Tessera server; it is the address you also use to open Tessera in the browser.",
+      "tray": "If you close the window, Tessera keeps running in the notification area; use the icon there to reopen the window, enable automatic start, or quit the app.",
+      "update": "When a newer version is available the app notifies you and brings you to this page.",
+      "versionLabel": "Current version: {version}",
+      "channelBeta": "Beta build, commit {commit}",
+      "downloadWindows": "Download for Windows",
+      "downloadLinux": "Download for Linux",
+      "fileInfo": "{name} · {size}",
+      "unavailable": "No desktop packages are available on this server yet."
+    },
     "sourceDeleteConfirm": "Are you sure you want to delete this calendar source?",
     "sourceDeleteCta": "Delete source",
     "providerDeleteConfirm": "Are you sure you want to delete this search provider?",