Fix newline handling when appending to .env

[MikeGarde]

• Ensure appending a new entry inserts a leading newline if the existing content lacks a trailing one, so the new key starts on its own line.
• Extend tests/set_and_delete.rs to cover the newline-edge case: when the file has no trailing newline, adding a new key appends on its own line and the final content ends with a newline; verify both keys are readable via --file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/handlers/set_value.rs	100.00% <100.00%> (ø)

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[MikeGarde]

Closes #36