panel-redesign: the application form is one POST, under its domain

Stage 1, step 4 (plan § Routes).

Service (internal/app)
  - Settings{Mode, Addresses, AuthIPRestrict, AuthAllowedIPs, Limit} is what
    the form holds besides the login. CreateWithSettings creates the
    application with all of it and removes it again if a later write fails;
    SaveSettings validates the sender rule first and, if a later write
    fails, puts back what it had already changed. The form produces a whole
    application or leaves things as they were.
  - Limit.Mode "" is "use the domain limit"; auto takes its ceiling from a
    recalculation the caller supplies.

Routes and handlers
  - GET, POST /outbound/domains/{id}/applications/new
    GET, POST /outbound/domains/{id}/applications/{aid}
    POST .../{aid}/password, .../{aid}/ratelimit/recalc, .../{aid}/delete
    replace POST /domains/{id}/applications and the six /applications/{aid}/*
    routes. lookupDomainApplication answers 404 when the application is not
    in the domain named by the path - also when the caller may open that
    domain.
  - A generated password is the response to its POST only (no GET path) and
    is sent with Cache-Control: no-store.
  - The login is not renamed by the form; a changed login is a 400.

Old domain page: the three per-application forms (mode, IP restriction,
limit) are one form with one Save; "Remove limit" became the "Use the domain
limit" choice. The e2e client follows (edited, not run here).

Tests: service - create and save in every limit mode, and all-or-nothing on
a refused sender rule, a failed recalculation and a failed sender-map
rebuild; handlers - the form's parsing, one POST creating sender + IPs +
limit, four kinds of refused form creating nothing, save, the no-store
header on both password pages, and five application routes against another
tenant's application addressed through its own domain and through the
caller's.

With @routes removed by hand TestRoutesFollowNavigation now reports only
GET /server/health (step 5).

go test ./...  green except the two Windows-only failures in internal/domain
and internal/logtail.

Roadmap 14/27.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/e2e/negative_test.go

@@ -26,7 +26,7 @@ func testLevel2RateLimit(t *testing.T, sc *scenario) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if err := sc.panel.setRateLimit(fmt.Sprintf("/applications/%s/ratelimit", appID), 1, 3600); err != nil {
+	if err := sc.panel.setApplicationLimit(sc.domainID, appID, 1, 3600); err != nil {
 		t.Fatalf("save application rate limit: %v", err)
 	}