/calendar/all is the same calendar page over all of them. A board is what
the app is organised by, but a due date is not — the card due on Tuesday is
due on Tuesday whichever board it is on, and reading five calendars to find
that out was the gap this closes.

The same component, not a second one: the month grid, the timeline, the
swipe and the entrance are all shared, and the scope decides only which
cards are loaded, what the legend's chips stand for and what the .ics links
ask for. It is read off the path, because /calendar/all is a literal segment
and cannot arrive as a route parameter — but it changes under a reused
component exactly as BoardId does, so it is handled beside it.

The category here is the board rather than the tag (CalendarTimeline.
BoardLanes, and the legend renders the boards): tags are per board, so five
boards' worth are five vocabularies that do not line up, and "which board is
this card from" is the question this view exists to answer and a tag cannot.
That keeps "the legend is the filter" true in both scopes. A board's colour
is derived from its id (BoardPalette) rather than stored — one view wanting
a colour is not a migration and an admin control — so two boards can share
one, and the legend, the lane and the tooltip name the board for that reason.

The board query on both .ics endpoints now carries the scope too
(CalendarScope): absent is the default board as before, `all` is every board
the viewer can see and stays dynamic, and `3,7` is exactly those. A named
board that has gone takes the whole feed with it rather than being dropped
from it, and a value that is none of those shapes is a 404 — the same
refusal of quiet substitution the single-board case already made.

Also fixes a pre-existing crash on /admin/boards: ReloadAsync published the
board list before _edits was filled, and ComponentBase renders after the
first await in OnInitializedAsync, so a slow second query made
_edits[board.Id] a KeyNotFoundException out of the prerender.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

@@ -333,0 +350,4 @@
+            var resolved = new List<Board>(scope.BoardIds.Count);
+            foreach (var id in scope.BoardIds)
+            {
+                if ((await boards.ResolveAsync(id, viewer, ct)).Board is not { } named) return null;