fix: handle null response after claude reset

[rasyid7]

Fix: Server returned invalid response #10

Summary of Changes

Modified ClaudeMeter to handle a specific API response case when the Claude session has been reset.

Problem

When Claude's session resets, the API returns resets_at: null for five_hour:

{
    "five_hour": {
        "utilization": 0.0,
        "resets_at": null
    },
    ...
}

Previously, this caused an error: MappingError.missingCriticalField(field: "fiveHour.resetsAt")
Solution

1. UsageLimit.swift - Made resetAt optional
   • Changed resetAt: Date → resetAt: Date?
   • resetDescription now returns "Starts when a message is sent" when nil
   • Updated isResetting and isAtRisk to handle nil safely
2. UsageAPIResponse.swift - Handle null reset dates
   • Session usage with resets_at: null → utilization forced to 0%
   • Same handling for optional sevenDaySonnet
3. UsageCardView.swift - Updated UI text
   • When resetAt is nil: shows "Starts when a message is sent"
   • When resetAt exists: shows "Resets in X hours"
4. NotificationServiceProtocol.swift & NotificationService.swift - Updated for optional dates
   • Changed resetTime: Date → resetTime: Date?
   • Notification body handles nil reset time gracefully
5. Test spy updated to match new protocol signature

[eddmann]

Thanks Rasyid. The null resets_at issue raised here has been addressed via #21, and your contribution has been acknowledged in that PR.

The final implementation follows the stricter version that still fails malformed non-null reset dates, while accepting truly null reset timestamps.

[eddmann]

Thanks again Rasyid. The null resets_at fix is now included in ClaudeMeter v1.3.1.

The released implementation accepts truly null reset timestamps after a usage window resets, while still treating malformed non-null reset dates as invalid responses. Your PR helped identify and validate the issue.