doesn't work with non-Bluesky PDSes

In sync-cmd.js, the app should look up user's assigned PDS to pass to CredentialManager from the DID document, instead of assuming it's bsky.social 😅

% bun leaflet-md sync            

 ERROR  AuthenticationRequired > Invalid identifier or password                                                                                                                                   

    at ok (node_modules/@atcute/client/dist/client.js:175:11)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async CredentialManager.login (node_modules/@atcute/client/dist/credential-manager.js:150:25)
    at async Object.run (node_modules/@sharpmars/leaflet-md/dist/cli.js:381:3)
    at async runCommand (node_modules/citty/dist/index.mjs:316:16)
    at async runCommand (node_modules/citty/dist/index.mjs:307:11)
    at async runMain (node_modules/citty/dist/index.mjs:445:7) 

Also, handler.session.pdsUri will be undefined for non-Bluesky PDSes, because in the response from createSession, the didDoc field is null, so that value should not be used as is to create a URL (the const carReq = line), but only taken if it's set, otherwise just use the previously used PDS hostname.

 % bun leaflet-md sync

 ERROR  Failed to parse URL from undefined/xrpc/com.atproto.sync.getRepo?did=did:plc:oio4hkxaop4ao4wz2pp3f4cr                                                                                     

    at node:internal/deps/undici/undici:15845:13
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async Object.run (node_modules/@sharpmars/leaflet-md/dist/cli.js:387:18)
    at async runCommand (node_modules/citty/dist/index.mjs:316:16)
    at async runCommand (node_modules/citty/dist/index.mjs:307:11)
    at async runMain (node_modules/citty/dist/index.mjs:445:7)

  [cause]: Invalid URL

      at new URL (node:internal/url:828:25)
      at new Request (node:internal/deps/undici/undici:10413:25)
      at fetch (node:internal/deps/undici/undici:11325:25)
      at fetch (node:internal/deps/undici/undici:15843:10)
      at fetch (node:internal/bootstrap/web/exposed-window-or-worker:83:12)
      at Object.run (node_modules/@sharpmars/leaflet-md/dist/cli.js:387:24)
      at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
      at async runCommand (node_modules/citty/dist/index.mjs:316:16)
      at async runCommand (node_modules/citty/dist/index.mjs:307:11)
      at async runMain (node_modules/citty/dist/index.mjs:445:7)