chore(deps): bump strum and strum_macros to 0.28

[dobby-coder]

Bumps strum and strum_macros from 0.27.2 to 0.28.0.

What

• Cargo.toml: strum and strum_macros 0.27.2 → 0.28.0
• Cargo.lock: refreshed

Why

Of the open major bumps tracked in #117 (askama, reqwest, sha2, rand, strum), this is the smallest API delta. Filed by Dobby per encryption4all/dobby#25.

Verification (locally)

• cargo build --release → succeeds
• cargo test --release → all tests pass (30+ in headers + integration suites)

Reviewer quickstart

git fetch origin && git checkout chore/strum-0.28 && cargo build --release && cargo test --release

The remaining majors (askama 0.15, reqwest 0.13, sha2 0.11, rand 0.10) all have larger API changes and need their own PRs — see #117.

Refs #117