refactor: use helper to derive variable path

fix: communicate missing charge threshold setting

Return a user-friendly message when the charging threshold setting does
not exist instead of panicking.

build: bump golang.org/x/sys from 0.15.0 to 0.16.0

Bumps [golang.org/x/sys](https://github.com/golang/sys) from 0.15.0 to 0.16.0.
- [Commits](golang/sys@v0.15.0...v0.16.0)

---
updated-dependencies:
- dependency-name: golang.org/x/sys
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

feat: replace dependency on Bash with /bin/sh for cross-platform comp…

…atibility

This ensures cross-platform compatibility and accessibility for a wider range of users.

Resolves: #68.

fix: restore persistence functionality

This commit addresses an issue where the switch to using systemd
templates and format specifiers introduced a regression, causing the
persistence functionality to no longer work as expected.

To resolve this issue, we have reverted to an earlier implementation
that manually writes all the units and uses Go's exec.LookPath to
resolve the shell path.

Resolves: #65

build: set lower minimum supported version

build: set lower minimum supported version

docs: update README

fix: fix uncaught insufficient permission error

Switching over errors does not work with wrapped errors. Prefer the more
verbose errors.Is instead.

refactor: update threshold and variable packages