Add bilingual UI and improve event actions layout

Add persistent German/English switching, translated UI messages and English 12-hour times. Align desktop event actions while preserving mobile layouts and record bilingual project guidance.

Include pending diary deletion exclusions. Validate both languages with nine tests.

app/i18n.py

@@ -0,0 +1,46 @@
+"""Request-local UI translations. User content and stored identifiers stay unchanged."""
+
+import json
+from contextvars import ContextVar
+from pathlib import Path
+from urllib.parse import urlencode, urlsplit
+
+
+LANGUAGE_COOKIE = "metalcircle_language"
+current_language = ContextVar("current_language", default="de")
+current_page = ContextVar("current_page", default="/")
+ENGLISH = json.loads((Path(__file__).parent / "locales" / "en.json").read_text())
+
+
+def gettext(message):
+    if not isinstance(message, str):
+        return message
+    return ENGLISH.get(message, message) if current_language.get() == "en" else message
+
+
+def format_time(value):
+    """Format display times without changing stored or form-submitted values."""
+    if current_language.get() == "en":
+        period = "AM" if value.hour < 12 else "PM"
+        return f"{value.hour % 12 or 12}:{value.minute:02d} {period}"
+    return value.strftime("%H:%M")
+
+
+def format_datetime(value):
+    return f"{value.strftime('%d.%m.%Y')} {format_time(value)}"
+
+
+def safe_return_path(value):
+    """Only allow local absolute paths, including their query string and fragment."""
+    if not value or not value.startswith("/") or value.startswith("//"):
+        return "/"
+    if "\\" in value or any(ord(char) < 32 or ord(char) == 127 for char in value):
+        return "/"
+    parsed = urlsplit(value)
+    if parsed.scheme or parsed.netloc or parsed.path.startswith("/language/"):
+        return "/"
+    return value
+
+
+def language_url(language):
+    return f"/language/{language}?{urlencode({'next': current_page.get()})}"