A deployment now holds any number of boards, with no per-board access
rights: every allowlisted member sees every board, which is the shape
issue #68 asked for.

A board owns its columns and its tags. Cards deliberately carry no board
of their own — the column they sit in already answers the question — so
reads take a board id and writes find it from the column or the card.

The rework:

- `Board` entity plus `BoardId` on `BoardColumn` and `Tag`, with the
  AddBoards migration handing an existing board's columns and tags to a
  board it creates for them. The name goes in as plaintext because SQL
  inside a migration has no encryptor; EncryptionMigrator, which runs
  straight after MigrateAsync, seals it.
- The old `BoardService` (cards and columns) is now `CardService`, and
  `BoardService` is the boards themselves — list, default, CRUD. A page
  component named `Board` would shadow the entity in every razor file,
  so `Board.razor` is `BoardPage.razor`; its route is unchanged.
- `BoardNotifier.BoardChanged` carries a board id, nullable for the
  mutations that really do touch every board (a member's profile), plus
  a `BoardsChanged` event for the set of boards itself.
- Which board is a route parameter, never circuit state: `/board/{id}`,
  `/calendar/board/{id}`, `/admin/columns/{id}`, `/admin/tags/{id}`. The
  bare paths mean the default board, so every existing link still works.
- Deleting a board never deletes a card: the FKs are Restrict, and the
  service refuses a board holding cards, and the last board outright.
- A new board is seeded with To Do / Doing / Done so it is usable at
  once. Tag name uniqueness is scoped to a board.
- The nav header's Board link becomes a switcher once there is a second
  board, and keeps you on the page you were on. `/admin/boards` manages
  them; the column sort interop is parameterised to serve both lists.
- The calendar, its export and its subscribable feed all take an
  optional `board`, 404 on one that is gone, and name the board in
  X-WR-CALNAME.

Verified: dotnet build, dotnet test (157 passed), and the e2e suite
driven against a live app with Postgres — tests/e2e/boards.spec.js (6
new tests) plus board, calendar, filter, mobile, i18n and prefs specs.
The AddBoards backfill was applied to a database seeded at the previous
migration and checked row by row.

@@ -124,0 +138,4 @@
+        var board = new Board { Name = "Board", Position = 1.0, CreatedAt = DateTime.UtcNow };
+        db.Boards.Add(board);
+        await db.SaveChangesAsync(ct);
+

@@ -84,3 +103,3 @@
 
-    private static void ValidateCardContent(string title, string descriptionMarkdown)
+    public async Task<Board> CreateAsync(string name, CancellationToken ct = default)
     {