refactor(updater): remove hex dependency

[StudentWeis]

Summary

Remove the updater module's dependency on the external hex crate by encoding SHA-256 digests locally.

Linked Issue

Closes #118

Changes

• replace hex::encode in the updater checksum path with a small local lowercase hex encoder
• remove the unused hex dependency from the crate manifest and lockfile

Testing

• scripts/precheck.sh passes locally
• New / updated tests cover the change
• existing compute_sha256_hex_reads_file_content_expected test continues to validate the exact checksum output

Self-Check

• PR title follows Conventional Commits
• No hardcoded user-facing strings — i18n keys added to all locale files
• New UI components use gpui-component
• Errors defined with thiserror (no manual impl Display/Error)
• No unrelated changes mixed in