Fix TUI bugs: folder selection, filter stability, UI consistency

- Mail: Fix folder/account selector not triggering reload (use direct
  fetch instead of reactive reload_needed flag)
- Tasks: Store all_projects/all_tags on mount so filters don't change
  when filtering; add OR search for multiple tags
- Sync: Use rounded borders and border_title for sidebar/activity log
- Calendar: Remove padding from mini-calendar, add rounded border and
  border_title to invites panel

src/calendar/widgets/InvitesPanel.py

@@ -38,6 +38,7 @@ class InvitesPanel(Widget):
         height: auto;
         min-height: 3;
         padding: 0 1;
+        border: round $primary;
     }
     """
 
@@ -71,6 +72,10 @@ class InvitesPanel(Widget):
         if invites:
             self.invites = invites
 
+    def on_mount(self) -> None:
+        """Set border title on mount."""
+        self.border_title = "Invites"
+
     def _get_theme_color(self, color_name: str) -> str:
         """Get a color from the current theme."""
         try:

src/calendar/widgets/MonthCalendar.py

@@ -62,7 +62,7 @@ class MonthCalendar(Widget):
     MonthCalendar {
         width: 24;
         height: auto;
-        padding: 0 1;
+        padding: 0;
     }
     """