fix(storage): refuse to open a workspace whose database file is missing

With .ykanban present but ykanban.db gone, opening used to create an empty
database and show an empty board without any hint. It now throws
WorkspaceDatabaseMissingException and shows the open-failed page, telling the
user to restore a backup or delete .ykanban and initialize again.

USAGE.md

@@ -62,7 +62,7 @@ instead:
 | Folder not found | `<path>` does not exist or is not a folder. |
 | No workspace here | `<path>` exists but has no `.ykanban`. The **Initialize workspace** button creates one, with the columns To Do / In Progress / Done (named in the current interface language). |
 | Workspace locked | Another YKanBan window already has this workspace open. Close that window first. Also shown when another window takes the lock while you initialize. |
-| Cannot open workspace | The database was created by a newer YKanBan, is damaged or cannot be opened, or a file-system or permission error occurred (also when initializing fails for a reason other than the lock). The English error details can be selected and copied. |
+| Cannot open workspace | `.ykanban` exists but `ykanban.db` in it is missing (restore it from a backup, or delete `.ykanban` and initialize again; YKanBan never silently creates an empty board), the database was created by a newer YKanBan, is damaged or cannot be opened, or a file-system or permission error occurred (also when initializing fails for a reason other than the lock). The English error details can be selected and copied. |
 
 The window title is `<workspace name> - YKanBan` while a workspace is open and
 just `YKanBan` on these pages.