feat(projects): change a member's role from the member list

An invite sent with the wrong role could not be corrected. The only way
back was to remove the person and invite them again — and removing a member
unassigns every task they were responsible for, so fixing a dropdown cost
somebody their work. This is the other side of "an external user cannot
create tasks": the rule was right, the role was wrong, and there was no way
to say so.

PATCH against a membership that already exists, not the upsert AddMember
does. Somebody who is not a member is not silently made one: that is an
invite, it belongs to the invite flow, and doing it here would let a typo
in a user id add a stranger to a project without anybody being asked.

The owner is refused, with the real reason rather than the true but useless
one. Ownership lives on the project rather than in project_members, so the
plain answer would have been "not a member" — which is correct and tells
nobody anything. Ownership is transferred, not granted, and two owners is a
state with no way to settle a disagreement about who may remove whom.

In the list, the owner's standing is text and everybody else's is a
dropdown. A disabled control would be a thing that exists to say no.

And the roles are finally in Danish. The API's words are `owner`, `editor`
and `viewer`, and they were being printed straight into a Danish page; the
list now uses the same three phrases the invite form beside it already did.

The smoke test drives both browsers, because the interesting part is that
the other one's permissions really change: a viewer is told the project is
read-only and has no quick-add box, the owner promotes them from the list,
and after a reload the box is there and the notice is gone.

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

internal/store/projects.go

@@ -389,6 +389,33 @@ func (db *DB) AddMember(ctx context.Context, projectID, userID string, role Role
 	return err
 }
 
+// SetMemberRole changes what an existing member may do.
+//
+// An UPDATE rather than the upsert AddMember does, and the difference is the
+// point: this is a PATCH against a membership that exists. Somebody who is not a
+// member is not silently made one — that is an invite, it belongs to the invite
+// flow, and doing it here would let a typo in a user id add a stranger to a
+// project without anybody being asked.
+//
+// Owner is refused for the same reason AddMember refuses it: ownership is
+// transferred, not granted, and two owners is a state with no way to settle a
+// disagreement about who may remove whom.
+func (db *DB) SetMemberRole(ctx context.Context, projectID, userID string, role Role) error {
+	if !role.Valid() || role == RoleOwner {
+		return errors.New("store: members may be editor or viewer")
+	}
+	res, err := db.ExecContext(ctx,
+		`UPDATE project_members SET role = ? WHERE project_id = ? AND user_id = ?`,
+		string(role), projectID, userID)
+	if err != nil {
+		return err
+	}
+	if n, _ := res.RowsAffected(); n == 0 {
+		return ErrNotFound
+	}
+	return nil
+}
+
 func (db *DB) RemoveMember(ctx context.Context, projectID, userID string) error {
 	return db.Tx(ctx, func(tx *sql.Tx) error {
 		// Anything assigned to them is unassigned rather than left pointing at