## Summary
**What is the goal of this PR?**
This change avoids the pattern of creating a `std::string` using
`.substr` in order to compare against a file extension literal.
```c++
std::string path;
if (path.length() >= 4 && path.substr(path.length() - 4) == ".ext")
```
The `checkFileExtension` utility has moved from StringUtils to
FsHelpers, to be available to code in lib/. The signature now accepts a
`std::string_view` instead of `std::string`, which makes the single
implementation reusable for Arduino `String`.
Added utility functions for commonly repeated extensions.
These changes **save about 2 KB of flash (5,999,427 to 5,997,343)**.
---
### AI Usage
While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.
Did you use AI tools to help write this code? _**NO**_
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