fix(auth): judge the passkey nudge by the configured origin

REVIEW-001 (M0) L7c: shouldNudge trusted the request Host, so a reverse
proxy that rewrites Host wrongly suppressed (or enabled) the nudge.

Prefer the configured server.base_url host (Deps.BaseURL) and fall back to
the request Host only when it is unset. Wire the secure session-cookie name
through SessionCookieNameFor so the `__Host-` prefix (L1s) is used
consistently by the flows.

internal/core/auth/flow/flow.go

@@ -25,6 +25,8 @@ import (
 	"fmt"
 	"log/slog"
 	"net/http"
+	"net/url"
+	"strings"
 	"time"
 
 	"git.lohmar.co.uk/cclohmar/NextMyDay/internal/core/audit"
@@ -71,6 +73,10 @@ type Deps struct {
 	// (SPEC-006 §8). The door wires it to setup mode, where a passkey is
 	// useless. nil = the nudge runs (subject to the other conditions).
 	SuppressNudge func() bool
+	// BaseURL is the CONFIGURED public origin. It decides the nudge's origin
+	// signal: a proxy that rewrites `Host` must not flip the decision (L7c).
+	// Empty falls back to the request `Host`.
+	BaseURL string
 	// Secure marks cookies Secure (https).
 	Secure bool
 	// Logger receives structured logs. Defaults to slog.Default().
@@ -385,12 +391,12 @@ func (a *Actions) LoginFinish(w http.ResponseWriter, r *http.Request, guid strin
 // Logout revokes the current session and clears the cookie (SPEC-006 §5).
 func (a *Actions) Logout(w http.ResponseWriter, r *http.Request, guid string) error {
 	ctx := r.Context()
-	if c, err := r.Cookie(httpx.SessionCookieName); err == nil && c.Value != "" {
+	if c, err := r.Cookie(httpx.SessionCookieNameFor(a.d.Secure)); err == nil && c.Value != "" {
 		if err := a.d.Sessions.Revoke(ctx, c.Value); err != nil {
 			a.d.Logger.Warn("flow: logout revoke failed", "error", err)
 		}
 	}
-	httpx.ClearSessionCookie(w, httpx.SessionCookieName, a.d.Secure)
+	httpx.ClearSessionCookie(w, httpx.SessionCookieNameFor(a.d.Secure), a.d.Secure)
 	http.Redirect(w, r, "/home", http.StatusSeeOther)
 	return nil
 }
@@ -481,7 +487,7 @@ func (a *Actions) establishSessionAt(w http.ResponseWriter, r *http.Request, idn
 	if err != nil {
 		return err
 	}
-	httpx.SetSessionCookie(w, httpx.SessionCookieName, sess.ID, a.d.Secure)
+	httpx.SetSessionCookie(w, httpx.SessionCookieNameFor(a.d.Secure), sess.ID, a.d.Secure)
 	if target == "" {
 		target = a.successTarget(idn.GUID)
 	}
@@ -512,7 +518,7 @@ func (a *Actions) shouldNudge(ctx context.Context, r *http.Request, idn auth.Ide
 	if a.d.SuppressNudge != nil && a.d.SuppressNudge() {
 		return false
 	}
-	if httpx.IsLoopbackHost(r.Host) {
+	if httpx.IsLoopbackHost(a.originHost(r)) {
 		return false
 	}
 	if a.d.Users == nil {
@@ -534,6 +540,16 @@ func (a *Actions) shouldNudge(ctx context.Context, r *http.Request, idn auth.Ide
 	return !hasPasskey
 }
 
+// originHost is the origin the nudge is judged against (L7c): the CONFIGURED
+// base_url's host when set — so a reverse proxy that rewrites `Host` cannot
+// wrongly suppress (or enable) the nudge — falling back to the request `Host`.
+func (a *Actions) originHost(r *http.Request) string {
+	if u, err := url.Parse(strings.TrimSpace(a.d.BaseURL)); err == nil && u.Host != "" {
+		return u.Host
+	}
+	return r.Host
+}
+
 // ── the passkey nudge (SPEC-006 §8, D-041) ──────────────────────────────────
 
 // PasskeyPrompt renders the passkey nudge screen. It is a session-gated ROUTE
@@ -646,7 +662,7 @@ func (a *Actions) issueSession(r *http.Request, idn auth.Identity) (auth.Session
 	if m, ok := audit.MetaFrom(ctx); ok {
 		ip = m.ClientIP
 	}
-	if c, err := r.Cookie(httpx.SessionCookieName); err == nil && c.Value != "" {
+	if c, err := r.Cookie(httpx.SessionCookieNameFor(a.d.Secure)); err == nil && c.Value != "" {
 		if old, err := a.d.Sessions.Validate(ctx, c.Value); err == nil && old.IdentityID == idn.ID {
 			return a.d.Sessions.Rotate(ctx, c.Value)
 		}