fix: add error handling for WiFi.scanNetworks() in /scan endpoint

[avineet4]

Description

This PR fixes missing error handling in the /scan endpoint when WiFi.scanNetworks() fails.

Problem

• WiFi.scanNetworks() can return negative values on error (e.g., -1 for WIFI_SCAN_FAILED, -2 for WIFI_SCAN_RUNNING)
• The code doesn't check for negative values before iterating through results
• When scan fails, client receives empty array [] with 200 OK status, masking the error
• No memory cleanup (WiFi.scanDelete()) is performed
• Serial output doesn't indicate scan failure, making debugging difficult

Solution

• Added error checking for networks < 0
• Return 500 error with JSON error response when scan fails
• Added WiFi.scanDelete() to prevent memory leak
• Improved serial output to indicate scan success/failure
• Proper error reporting to client instead of masking errors

Changes Made

• Modified /scan endpoint in src/EasyWiFi.cpp (lines 300-326)
• Added error handling with appropriate HTTP status codes
• Added memory cleanup for scan results
• Improved logging for better debugging

Testing

• ✅ Code compiles without errors
• ✅ No linting issues introduced
• Manual testing recommended: Force scan failure by testing during WiFi busy state

Impact

• Low severity but improves user experience
• Users now see proper error messages instead of empty network lists
• Prevents potential memory leaks
• Makes debugging connectivity issues easier

Fixes #10