feat: add cloud download quota display and local session cache

[lnzx]

1. display cloud download quota

• Add CloudDownload field to Quotas struct
• Add Remaining() method to Quota struct to calculate remaining quota
• GetQuota() now returns full QuotaMessage including Quotas
• Display cloud download quota (total/used/remaining) in quota command output

Output Example

$ ./pikpakcli quota
Storage:
total used
6442450944 0

cloud download:
total used remaining
5 5 0

$ ./pikpakcli quota -H
Storage:
total used
6.0GB 0B

cloud download:
total used remaining
5 5 0

2. Session Cache

Every command invocation triggers a full login request, including the
captcha/init endpoint. This is slow and risks account suspension due
to frequent authentication requests.

Add local session cache in ~/.config/pikpakcli/session_{md5}.json:

• Reuse valid access_token directly
• Use refresh_token to renew expired access_token
• Fall back to full login only when refresh_token is also invalid

Session file is named by account md5 hash to support multiple accounts.

Changes

• session.go: add sessionFile, saveSession, loadSession,
  isTokenExpired
• pikpak.go: refactor Login() to check local session first,
  extract original login logic to login()
• refresh_token.go: replace p.Login() with p.login() to avoid
  circular calls