fix: handle 401 gracefully on web; add Bearer token auth for Tauri

- fetchMe() no longer surfaces 401 as a user-facing error (it just
  means 'no session', not a failure)
- API client auto-clears auth state on 401 mid-session so the user
  gets redirected to login instead of seeing 'ERR: Request failed: 401'
- Session middleware now accepts Authorization: Bearer <token> header
  as fallback when no cookie is present (for Tauri/native clients)
- Login, register, and WebAuthn endpoints expose X-Session-Token header
  so non-browser clients can capture the token

internal/auth/handlers.go

@@ -331,6 +331,7 @@ func (h *Handler) Register(w http.ResponseWriter, r *http.Request) {
 	}
 
 	h.setSessionCookie(w, token)
+	w.Header().Set("X-Session-Token", token)
 	w.Header().Set("Content-Type", "application/json")
 	json.NewEncoder(w).Encode(map[string]string{"id": userID})
 }
@@ -383,6 +384,7 @@ func (h *Handler) Login(w http.ResponseWriter, r *http.Request) {
 	}
 
 	h.setSessionCookie(w, token)
+	w.Header().Set("X-Session-Token", token)
 	w.Header().Set("Content-Type", "application/json")
 	json.NewEncoder(w).Encode(map[string]string{"id": userID})
 }

internal/auth/webauthn.go

@@ -346,6 +346,7 @@ func (h *WebAuthnHandler) LoginFinish(w http.ResponseWriter, r *http.Request) {
 		Secure:   true,
 	})
 
+	w.Header().Set("X-Session-Token", token)
 	w.Header().Set("Content-Type", "application/json")
 	json.NewEncoder(w).Encode(map[string]string{"status": "authenticated"})
 }