fix(storage): enforce restrictive file permissions on clipboard history (fixes #47)

[riddhima25bet10005-a11y]

Summary

This PR addresses the critical issue of plaintext clipboard history being stored with world-readable permissions by explicitly setting restrictive Unix file permissions.

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that changes existing behaviour)
• 📚 Documentation update
• 🧪 Tests (adding or improving test coverage)
• ♻️ Refactor (no functional changes)
• 🔧 Chore (dependency update, build tooling, CI)

Related Issue

Closes #47

Changes Made

• Updated atomic_write in src/storage/disk.rs to set the file permission mode to 0600 (user read/write only) on newly created files using std::os::unix::fs::OpenOptionsExt.
• Updated flush_force in src/storage/disk.rs to set the directory permission mode to 0700 (user read/write/execute only) on the ~/.clipwallet/store directory using std::os::unix::fs::PermissionsExt.

Testing Done

• cargo fmt — code is formatted
• cargo clippy -- -D warnings — no Clippy warnings (Skipped: macOS-only project compiling on Windows)
• cargo test — all tests pass (Skipped: macOS-only project compiling on Windows)
• Manually tested on macOS (version: _________)
• Tested the daemon lifecycle (clipwallet run → hotkeys → clipwallet stop)
• Tested relevant hotkey chords end-to-end

Checklist

• My branch is up to date with main (git fetch upstream && git rebase upstream/main)
• My commits follow the Conventional Commits format
• I have updated relevant documentation (README, inline docs, CHANGELOG) if needed
• I have not logged clipboard contents anywhere, even at debug level
• This PR is focused — it addresses one issue or feature

[riddhima25bet10005-a11y]

Hi @shaaravraghu
Kindly review and approve.

[riddhima25bet10005-a11y]

Hi @shaaravraghu
Kindly review.