betterbird: never lose an unsent message, never answer an address, and keep the test apart (review of #155)

A restart ended Betterbird with whatever a composer held; now it refuses while a Msgcompose window is
open unless discard_unsent says the operator agreed, and answers within the runtime's 30 s. A
preference's value could carry an address, a user%40domain@host URI, a host or the home; those are
masked, recent-folder, account and cloud-file preferences and every add-on preference but the theme's
id are withheld, and a request for everything is refused. A log kept the token after Bearer or Basic.
The reminder test's Betterbird counted as the operator's and left its profile behind when it ended on
its own; it is now left out everywhere and its unit removes the profile when it stops.

modules/betterbird/cmd/betterbird-tools/main.go

@@ -100,11 +100,21 @@ func tools() []stdio.Tool {
 		},
 		{
 			Name: "betterbird_restart",
-			Description: "End Betterbird (asked to quit, forced after 20 s), place the module's user.js when the profile " +
+			Description: "End Betterbird (asked to quit, forced after 10 s), place the module's user.js when the profile " +
 				"allows it, and start Betterbird again in the operator's desktop session under the account's service " +
-				"manager (unit mesh-betterbird), its output in the journal. An unsent message in a composer is lost: ask " +
-				"the operator first. Answers the pids ended and the new main process. (a)",
-			Run: func(map[string]any) (any, error) { return machine.Restart() },
+				"manager (unit mesh-betterbird), its output in the journal. Refused while a message is being written " +
+				"(a composer window is open), because the unsent message would be lost, unless discard_unsent is true — " +
+				"ask the operator first. Answers the pids ended and the new main process. (a)",
+			Input: map[string]any{
+				"discard_unsent": map[string]any{"type": "boolean", "description": "restart even with a composer open, losing what it holds (default false)"},
+			},
+			Run: func(args map[string]any) (any, error) {
+				discard, err := flag(args, "discard_unsent", false)
+				if err != nil {
+					return nil, err
+				}
+				return machine.Restart(discard)
+			},
 		},
 		{
 			Name: "betterbird_check",