feat: Phase 0 – Gerüst (FastAPI+SQLite, Multi-User-Login, React-App-Shell, Deploy)

Grundgerüst für Certain Finance:
- Backend: FastAPI + SQLAlchemy + SQLite, Alembic-Migrationen (Batch-Mode),
  argon2-Passwort-Hashing, server-seitige Sitzungen im Cookie, Erst-Admin,
  Benutzerverwaltung mit Temporär-Passwort, Einstellungen je Benutzer.
- Ein Prozess, ein Port: uvicorn liefert API (/api) und gebautes SPA aus,
  inkl. SPA-Deeplink-Fallback.
- Frontend: React 18 + Vite + JSX, Design-Tokens (Light/Dark), i18n DE/EN,
  Inline-SVG-Icons (keine Emojis), Login, Passwort-ändern, Dashboard-Platzhalter,
  Benutzer- und Einstellungsseiten.
- Deploy: nativer LXC-Installer + Update-Skript (systemd), env-Vorlage, Doku.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/locales/de.json

@@ -0,0 +1,85 @@
+{
+  "common": {
+    "save": "Speichern",
+    "cancel": "Abbrechen",
+    "delete": "Löschen",
+    "create": "Anlegen",
+    "edit": "Bearbeiten",
+    "loading": "Lädt…",
+    "logout": "Abmelden",
+    "admin": "Administrator",
+    "user": "Benutzer",
+    "close": "Schließen",
+    "retry": "Erneut versuchen",
+    "actions": "Aktionen"
+  },
+  "nav": {
+    "overview": "Übersicht",
+    "accounts": "Konten",
+    "categories": "Kategorien",
+    "transactions": "Buchungen",
+    "budget": "Budget",
+    "import": "Import",
+    "system": "System",
+    "users": "Benutzer",
+    "settings": "Einstellungen"
+  },
+  "login": {
+    "subtitle": "Bitte anmelden",
+    "username": "Benutzername",
+    "password": "Passwort",
+    "submit": "Anmelden",
+    "error": "Benutzername oder Passwort falsch",
+    "expired": "Deine Sitzung ist abgelaufen. Bitte erneut anmelden."
+  },
+  "changepw": {
+    "title": "Passwort ändern",
+    "forcedHint": "Bitte lege ein eigenes Passwort fest, bevor du fortfährst.",
+    "current": "Aktuelles Passwort",
+    "new": "Neues Passwort",
+    "repeat": "Neues Passwort wiederholen",
+    "submit": "Passwort speichern",
+    "mismatch": "Die neuen Passwörter stimmen nicht überein.",
+    "saved": "Passwort geändert."
+  },
+  "users": {
+    "title": "Benutzer",
+    "new": "Neuer Benutzer",
+    "username": "Benutzername",
+    "displayName": "Anzeigename",
+    "password": "Temporär-Passwort",
+    "passwordHint": "Der Benutzer wird beim ersten Login zum Ändern aufgefordert.",
+    "role": "Rolle",
+    "status": "Status",
+    "active": "Aktiv",
+    "inactive": "Deaktiviert",
+    "resetPassword": "Passwort zurücksetzen",
+    "deleteConfirm": "Diesen Benutzer wirklich löschen?",
+    "created": "Benutzer angelegt.",
+    "you": "(du)"
+  },
+  "settings": {
+    "title": "Einstellungen",
+    "baseCurrency": "Hauptwährung",
+    "baseCurrencyHint": "Währung für Budget-Summen und Dashboard-Gesamtwerte.",
+    "locale": "Sprache",
+    "theme": "Darstellung",
+    "themeSystem": "Automatisch (System)",
+    "themeLight": "Hell",
+    "themeDark": "Dunkel",
+    "budgetStartDay": "Budget-Monat beginnt am",
+    "budgetStartDayHint": "Tag, an dem dein Budget-Monat startet (z. B. 25.).",
+    "dateFormat": "Datumsformat",
+    "saved": "Gespeichert.",
+    "languageDe": "Deutsch",
+    "languageEn": "Englisch"
+  },
+  "dashboard": {
+    "title": "Übersicht",
+    "empty": "Noch keine Karten. Das Dashboard wird in einer späteren Phase ausgebaut."
+  },
+  "server": {
+    "unreachable": "Server nicht erreichbar.",
+    "sessionKept": "Deine Anmeldung ist noch gültig – sobald der Server wieder antwortet, geht es weiter."
+  }
+}

web/src/locales/en.json

@@ -0,0 +1,85 @@
+{
+  "common": {
+    "save": "Save",
+    "cancel": "Cancel",
+    "delete": "Delete",
+    "create": "Create",
+    "edit": "Edit",
+    "loading": "Loading…",
+    "logout": "Log out",
+    "admin": "Administrator",
+    "user": "User",
+    "close": "Close",
+    "retry": "Try again",
+    "actions": "Actions"
+  },
+  "nav": {
+    "overview": "Overview",
+    "accounts": "Accounts",
+    "categories": "Categories",
+    "transactions": "Transactions",
+    "budget": "Budget",
+    "import": "Import",
+    "system": "System",
+    "users": "Users",
+    "settings": "Settings"
+  },
+  "login": {
+    "subtitle": "Please sign in",
+    "username": "Username",
+    "password": "Password",
+    "submit": "Sign in",
+    "error": "Wrong username or password",
+    "expired": "Your session has expired. Please sign in again."
+  },
+  "changepw": {
+    "title": "Change password",
+    "forcedHint": "Please set your own password before continuing.",
+    "current": "Current password",
+    "new": "New password",
+    "repeat": "Repeat new password",
+    "submit": "Save password",
+    "mismatch": "The new passwords do not match.",
+    "saved": "Password changed."
+  },
+  "users": {
+    "title": "Users",
+    "new": "New user",
+    "username": "Username",
+    "displayName": "Display name",
+    "password": "Temporary password",
+    "passwordHint": "The user will be asked to change it on first login.",
+    "role": "Role",
+    "status": "Status",
+    "active": "Active",
+    "inactive": "Disabled",
+    "resetPassword": "Reset password",
+    "deleteConfirm": "Really delete this user?",
+    "created": "User created.",
+    "you": "(you)"
+  },
+  "settings": {
+    "title": "Settings",
+    "baseCurrency": "Base currency",
+    "baseCurrencyHint": "Currency for budget totals and dashboard aggregates.",
+    "locale": "Language",
+    "theme": "Appearance",
+    "themeSystem": "Automatic (system)",
+    "themeLight": "Light",
+    "themeDark": "Dark",
+    "budgetStartDay": "Budget month starts on",
+    "budgetStartDayHint": "Day your budget month starts (e.g. the 25th).",
+    "dateFormat": "Date format",
+    "saved": "Saved.",
+    "languageDe": "German",
+    "languageEn": "English"
+  },
+  "dashboard": {
+    "title": "Overview",
+    "empty": "No cards yet. The dashboard will be built out in a later phase."
+  },
+  "server": {
+    "unreachable": "Server unreachable.",
+    "sessionKept": "Your session is still valid – once the server responds again, it will continue."
+  }
+}