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

web/src/stores/auth.ts

@@ -151,12 +151,19 @@ export const useAuthStore = create<AuthState>((set) => ({
       set({ user, isAuthenticated: true, isLoading: false });
       autoSubscribePush();
     } catch (error) {
+      // 401 from /auth/me simply means "no active session" — not a
+      // user-facing error.  Only surface non-auth failures.
+      const isAuthError =
+        error instanceof Error && (error as import('../lib/api.ts').ApiError).status === 401;
       set({
         user: null,
         isAuthenticated: false,
         isLoading: false,
-        error:
-          error instanceof Error ? error.message : "Failed to fetch user",
+        error: isAuthError
+          ? null
+          : error instanceof Error
+            ? error.message
+            : "Failed to fetch user",
       });
     }
   },