fix(webapp): prevent TOCTOU race and use atomic writes

[lostmygithubaccount]

Summary

• Hold Mutex across entire load-modify-save cycle via modify_config() helper to prevent concurrent requests from overwriting each other's changes
• Write config to temp file then fs::rename for crash-safe atomic saves in toml_storage.rs
• Refactor all mutation handlers (add_url, add_group, delete_url, delete_group, edit_url, edit_group) to use new modify_or_err pattern

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