Encapsulate fsBytesUsed/fsBytesTotal with read-only accessors

[netmindz]

Summary

Part of an ongoing pass on WLED_GLOBAL declarations (see #5777-#5788, #5774/#12 for earlier ones). fsBytesUsed/fsBytesTotal are written and read by file.cpp (via updateFSInfo()), and read-only from json.cpp (status report in serializeInfo()).

Since json.cpp never writes them, moved both to file-local static in file.cpp and exposed two by-value getters — getFsBytesUsed()/getFsBytesTotal() — instead of a mutable reference.

This is deliberately narrower in scope than earlier Tier-2 work: only applied where the external file (json.cpp) is genuinely read-only. Because the getters return by value, an accidental write from json.cpp (or any other file) is now a build error instead of a silent bug — a real, compiler-enforced improvement, not just a rename. Other Tier-2 candidates where the external file legitimately needs to write (e.g. loadLedmap, dnsServer) are being left as plain globals, since no accessor shape actually restricts a write in C++ without a real module/friend boundary — wrapping those would just relabel the same coupling.

No behavior change — purely a storage/access-pattern change.

Test plan

• esp32dev: builds and links cleanly via pio run -e esp32dev.
• usermods env (builds all 59 usermods): builds and links cleanly via pio run -e usermods.
• Repo-wide grep confirms no remaining raw references to either identifier outside file.cpp.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between 0869ef4 and b01cb54.

📒 Files selected for processing (4)

• wled00/fcn_declare.h
• wled00/file.cpp
• wled00/json.cpp
• wled00/wled.h

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