Add context filter to Tasks TUI and fix calendar UI bugs

Tasks TUI:
- Add context support to TaskBackend interface (get_context, set_context,
  get_contexts methods)
- Implement context methods in DstaskClient
- Add Context section to FilterSidebar (above projects/tags)
- Context changes persist via backend CLI

Calendar TUI:
- Remove duplicate header from InvitesPanel (use border_title instead)
- Fix border_title color to use $primary
- Fix WeekGrid to always scroll to work day start (7am) on mount

src/calendar/widgets/InvitesPanel.py

@@ -39,6 +39,7 @@ class InvitesPanel(Widget):
         min-height: 3;
         padding: 0 1;
         border: round $primary;
+        border-title-color: $primary;
     }
     """
 
@@ -74,7 +75,12 @@ class InvitesPanel(Widget):
 
     def on_mount(self) -> None:
         """Set border title on mount."""
-        self.border_title = "Invites"
+        self._update_border_title()
+
+    def _update_border_title(self) -> None:
+        """Update border title with invite count."""
+        count = len(self.invites)
+        self.border_title = f"Invites ({count})" if count else "Invites"
 
     def _get_theme_color(self, color_name: str) -> str:
         """Get a color from the current theme."""
@@ -97,25 +103,21 @@ class InvitesPanel(Widget):
         return fallbacks.get(color_name, "white")
 
     def get_content_height(self, container, viewport, width: int) -> int:
-        """Calculate height: header + invite rows."""
-        # Header (1) + invites (2 lines each) + empty message if no invites
+        """Calculate height: invite rows only (no internal header)."""
         if not self.invites:
-            return 2  # Header + "No pending invites"
-        return 1 + len(self.invites) * 2  # Header + 2 lines per invite
+            return 1  # "No pending invites"
+        return len(self.invites) * 2  # 2 lines per invite
 
     def render_line(self, y: int) -> Strip:
         """Render a line of the panel."""
-        if y == 0:
-            return self._render_header()
-
         if not self.invites:
-            if y == 1:
+            if y == 0:
                 return self._render_empty_message()
             return Strip.blank(self.size.width)
 
         # Each invite takes 2 lines
-        invite_idx = (y - 1) // 2
-        line_in_invite = (y - 1) % 2
+        invite_idx = y // 2
+        line_in_invite = y % 2
 
         if 0 <= invite_idx < len(self.invites):
             return self._render_invite_line(
@@ -126,15 +128,6 @@ class InvitesPanel(Widget):
 
         return Strip.blank(self.size.width)
 
-    def _render_header(self) -> Strip:
-        """Render the panel header."""
-        primary_color = self._get_theme_color("primary")
-        count = len(self.invites)
-        header = f"Invites ({count})" if count else "Invites"
-        header = header[: self.size.width].ljust(self.size.width)
-        style = Style(bold=True, color=primary_color)
-        return Strip([Segment(header, style)])
-
     def _render_empty_message(self) -> Strip:
         """Render empty state message."""
         msg = "No pending invites"
@@ -179,6 +172,7 @@ class InvitesPanel(Widget):
         self.invites = invites
         if self.selected_index >= len(invites):
             self.selected_index = max(0, len(invites) - 1)
+        self._update_border_title()
         self.refresh()
 
     def select_next(self) -> None:
@@ -203,11 +197,11 @@ class InvitesPanel(Widget):
         """Handle mouse clicks."""
         y = event.y
 
-        if y == 0 or not self.invites:
+        if not self.invites:
             return
 
-        # Calculate which invite was clicked
-        invite_idx = (y - 1) // 2
+        # Calculate which invite was clicked (2 lines per invite)
+        invite_idx = y // 2
         if 0 <= invite_idx < len(self.invites):
             self.selected_index = invite_idx
             self.post_message(self.InviteSelected(self.invites[invite_idx]))