fix: support shared weekly limits for Grok Web Imagine

[chenyme]

Summary

• Accept Grok Web Imagine availability-only quota responses when windowSizeSeconds is present but remainingQueries is omitted.
• Treat missing product counters as shared weekly quota for confirmed Super/Heavy accounts.
• Keep Basic/Free accounts isolated from paid weekly quota fallback.
• Refresh both weekly usage and Imagine product availability after successful media requests.
• Preserve the previous quota snapshot when the paid weekly endpoint temporarily fails.
• Continue rejecting genuinely incomplete quota structures.

Why

Grok Web can return Imagine products such as imagePro with:

{
  "available": true,
  "windowSizeSeconds": 64800
}

This is a valid availability-only response for accounts governed by the shared weekly pool. The previous parser incorrectly required remainingQueries, causing quota synchronization to fail with:

Grok Web Imagine 配额字段 imagePro 结构不完整

Safety

• Explicit product quota windows take precedence over weekly fallback.
• available=false still installs an exhausted product window.
• Basic/Free accounts never inherit paid weekly quota.
• A failed weekly refresh cannot erase the last valid quota snapshot.
• Missing or invalid windowSizeSeconds remains an error.

Test plan

• Accept the Issue [Bug]: Grok Web Imagine 配额字段 imagePro 结构不完整 #972 availability-only payload.
• Reject availability-only products without a valid window size.
• Preserve the old snapshot when paid weekly synchronization fails.
• Verify Super/Heavy routing falls back to weekly.
• Verify Basic routing does not use weekly.
• Verify product-specific windows override weekly.
• Verify media finalization refreshes weekly and Imagine availability.
• go test -race for affected packages.
• go build ./...
• go vet ./...
• go test ./...

Fixes #972