Web: caret/selection fix, reminder options & labels, group mute, rounded modal

- Stop the blinking text caret / selection on non-editable UI chrome
  (headings, buttons, icons, dropdowns): body is user-select:none, with text
  selection opted back in for inputs/textareas. Felt like an editable field.
- Reminder options trimmed to 5/15/30 min, 1 h, 1 day, 1 week (dropped 10 min,
  2 h, 2 days; added 1 week). Labels now read "5 Minuten vorher", "1 Tag
  vorher", "1 Woche vorher" instead of "vor 5 Min." / "vor 1 Tag(en)".
- Group calendars can now be muted (reminder bell) in the sidebar when owned.
- Event modal corners rounded (20px) to match the pill buttons.
- version v45 -> v46.

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

frontend/css/app.css

@@ -44,6 +44,13 @@ body {
 }
 input, select, textarea, button { font-family: inherit; font-size: inherit; color: inherit; }
 button { cursor: pointer; border: none; background: none; }
+/* Treat the app like native UI chrome: clicking a heading, button, icon or
+   dropdown must not place a blinking text caret or select the label. Real text
+   entry (inputs / textareas) opts selection back in. */
+body { -webkit-user-select: none; user-select: none; }
+input, textarea, [contenteditable="true"], .selectable {
+  -webkit-user-select: text; user-select: text;
+}
 a { color: var(--primary); text-decoration: none; }
 ::-webkit-scrollbar { width: 6px; height: 6px; }
 ::-webkit-scrollbar-track { background: transparent; }
@@ -936,7 +943,7 @@ a { color: var(--primary); text-decoration: none; }
 .modal-card {
   background: var(--bg-surface);
   border: 1px solid var(--border);
-  border-radius: var(--radius);
+  border-radius: 20px;
   width: 100%;
   max-height: 90vh; overflow-y: auto;
   box-shadow: var(--shadow-lg);