Reject list saves when no SD card is available

[p-keminer]

Summary

• reject saved-list writes immediately when SD support is unavailable
• avoid reporting a successful save when no card is mounted

Testing

• Focused ESP32-C5 hardware test: saves without detected SD media were rejected immediately without a false success message.

Scope

• Later create, write, and flush failures are intentionally left to the transactional saved-list follow-up.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.40%. Comparing base (66f3c81) to head (daa5de7).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
esp32_marauder/WiFiScan.cpp	0.00%	21 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #1429      +/-   ##
==========================================
- Coverage     0.40%   0.40%   -0.01%     
==========================================
  Files           20      20              
  Lines        17176   17197      +21     
  Branches        11      11              
==========================================
  Hits            69      69              
- Misses       17103   17124      +21     
  Partials         4       4              

Flag	Coverage Δ
firmware-unit-tests	0.40% <0.00%> (-0.01%)	⬇️

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[Copilot]

Pull request overview

This PR improves saved-list reliability by failing fast when SD functionality is not available at runtime, preventing “successful save” messages when no SD media is mounted.

Changes:

• Add a shared helper to report SD-unavailable save failures via Serial (and screen when present).
• Gate saved-list “Save As” flows (AirTags/APs/SSIDs) on sd_obj.supported to reject writes early.

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[p-keminer]

All 22 firmware matrix builds and the Native Unity tests pass on this head. The remaining red statuses are limited to the repository-wide Codecov policy addressed separately in #1448.