feat(passwordless): add cookie challenge + low TTL

magic-link as now 2 modes:

        - invitation: long TTL (24), no challenge
        - passwordless login: short TTL (10min), cookie challenge to ensure that
        user goes back to same browser (no man in the middle capturing email)

src/interfaces/api/handlers/magic_link_handler.rs

@@ -25,13 +25,16 @@ use std::sync::Arc;
 
 use axum::{
     Router,
-    extract::{Path, State},
-    http::{HeaderValue, StatusCode, header::CONTENT_TYPE, header::LOCATION},
+    extract::{Path, Query, State},
+    http::{HeaderMap, HeaderValue, StatusCode, header::CONTENT_TYPE, header::LOCATION},
     response::{IntoResponse, Response},
     routing::get,
 };
+use serde::Deserialize;
 
-use crate::application::services::auth_application_service::MagicLinkRedemption;
+use crate::application::services::auth_application_service::{
+    MagicLinkRedeemResult, MagicLinkRedemption,
+};
 use crate::common::di::AppState;
 use crate::common::errors::ErrorKind;
 use crate::domain::entities::magic_link_token::MagicLinkResourceKind;
@@ -44,11 +47,21 @@ pub fn magic_link_routes() -> Router<Arc<AppState>> {
     Router::new().route("/magic/v1/{token}", get(redeem_magic_link))
 }
 
+#[derive(Debug, Deserialize)]
+struct RedeemQuery {
+    /// PR 22: `?confirm=1` means the user clicked the cross-browser
+    /// confirmation prompt's Continue button. The service skips the
+    /// challenge-cookie check on this re-entry.
+    #[serde(default)]
+    confirm: Option<String>,
+}
+
 #[utoipa::path(
     get,
     path = "/magic/v1/{token}",
     params(("token" = String, Path, description = "Opaque magic-link token")),
     responses(
+        (status = 200, description = "Cross-browser confirmation prompt (HTML page)"),
         (status = 302, description = "Redemption succeeded — redirects to the resource or to /#/sharedwithme"),
         (status = 410, description = "Token is unknown, expired, or already used"),
         (status = 503, description = "Magic-link feature is not configured on this server"),
@@ -58,6 +71,8 @@ pub fn magic_link_routes() -> Router<Arc<AppState>> {
 async fn redeem_magic_link(
     State(state): State<Arc<AppState>>,
     Path(token): Path<String>,
+    Query(query): Query<RedeemQuery>,
+    headers: HeaderMap,
 ) -> Response {
     let Some(auth_svc) = state.auth_service.as_ref() else {
         return error_page(
@@ -66,12 +81,35 @@ async fn redeem_magic_link(
         );
     };
 
+    // PR 22 browser binding: read the per-request challenge from the
+    // cookie (set by `POST /api/auth/magic-link/send` on the originating
+    // browser). The service compares it to the token's stored
+    // challenge. `confirm=1` means the user just clicked through the
+    // cross-browser prompt and is fine redeeming from a different
+    // browser anyway.
+    let incoming_challenge =
+        cookie_auth::extract_cookie_value(&headers, cookie_auth::MAGIC_REQUEST_COOKIE);
+    let cross_browser_confirmed = query
+        .confirm
+        .as_deref()
+        .map(|v| v == "1" || v == "true")
+        .unwrap_or(false);
+
     match auth_svc
         .auth_application_service
-        .redeem_magic_link(&token)
+        .redeem_magic_link(
+            &token,
+            incoming_challenge.as_deref(),
+            cross_browser_confirmed,
+        )
         .await
     {
-        Ok(redemption) => build_success_response(&state, redemption),
+        Ok(MagicLinkRedeemResult::Allowed(redemption)) => {
+            build_success_response(&state, *redemption)
+        }
+        Ok(MagicLinkRedeemResult::NeedsCrossBrowserConfirm) => {
+            cross_browser_confirmation_page(&token)
+        }
         Err(e) => {
             // Log the cause for ops; the user gets a generic page so the
             // outcome can't be used as an enumeration oracle.
@@ -113,10 +151,53 @@ fn build_success_response(state: &Arc<AppState>, redemption: MagicLinkRedemption
         state.core.config.auth.refresh_token_expiry_secs,
     );
     cookie_auth::append_csrf_cookie(response.headers_mut(), redemption.auth.expires_in);
+    // Clear the request-challenge cookie — it's single-use and we don't
+    // want a stale value on the browser confusing a later flow.
+    cookie_auth::append_clear_magic_request_cookie(response.headers_mut());
 
     response
 }
 
+/// Render the cross-browser confirmation page (PR 22). Shown when the
+/// magic-link token carries a `request_challenge` (login-via-email)
+/// but the inbound cookie didn't match — typically because the user
+/// requested the link from one browser and clicked it from another
+/// (phone vs desktop, work vs personal). The Continue button submits
+/// back to the same endpoint with `?confirm=1` so the service skips
+/// the challenge check and proceeds with redemption. Audit-logged at
+/// `magic_link.redeemed reason="cross_browser_confirmed"`.
+fn cross_browser_confirmation_page(token: &str) -> Response {
+    let confirm_url = format!("/magic/v1/{}?confirm=1", html_escape(token));
+    let body = format!(
+        "<!doctype html><html><head><meta charset=\"utf-8\">\
+         <title>Sign in — OxiCloud</title>\
+         <style>body{{font-family:system-ui,sans-serif;max-width:520px;margin:6em auto;\
+         padding:0 1em;color:#333;line-height:1.5}}\
+         h1{{font-size:1.4em}}.btn{{display:inline-block;padding:.7em 1.4em;\
+         background:#2563eb;color:#fff;border-radius:6px;text-decoration:none;\
+         font-weight:600;margin-top:1em}}.btn:hover{{background:#1d4ed8}}\
+         .note{{background:#fef3c7;border-left:3px solid #f59e0b;\
+         padding:.75em 1em;margin:1.5em 0;border-radius:4px;font-size:.95em}}</style>\
+         </head><body>\
+         <h1>Continue signing in on this device?</h1>\
+         <p>You opened this sign-in link in a different browser or device than \
+         the one where you requested it.</p>\
+         <p class=\"note\">If <strong>you</strong> requested this link, it's safe to continue. \
+         If you didn't request it, close this page — clicking Continue would sign \
+         someone else into your account.</p>\
+         <p><a class=\"btn\" href=\"{confirm_url}\">Continue and sign in</a></p>\
+         </body></html>",
+        confirm_url = confirm_url,
+    );
+
+    let mut response = (StatusCode::OK, body).into_response();
+    response.headers_mut().insert(
+        CONTENT_TYPE,
+        HeaderValue::from_static("text/html; charset=utf-8"),
+    );
+    response
+}
+
 /// Build the SPA hash-route the redemption should land on. Mirrors the
 /// front-end's `deserializeHash()` parser at `static/js/app/main.js`.
 ///