Add an export report button for completed desktop scans

[Muawiya-contact]

Goal

Let users export the completed scan report to a JSON file from the desktop app.

This helps users share a report, compare scans, or attach details to a GitHub issue without taking screenshots.

Suggested starting point

Frontend:

app/src/App.jsx

Tauri commands:

app/src-tauri/src/commands.rs

The backend already returns a serializable report:

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct Report {
    pub findings: Vec<Finding>,
    pub duplicate_sets: Vec<dedup::DuplicateSet>,
    pub total_reclaimable: u64,
    pub files_scanned: u64,
}

A first version can add an Export report button after a scan completes and write pretty JSON using a save dialog.

Acceptance criteria

• Export appears only after a completed report exists.
• Preview/in-progress reports are not exported as final reports.
• JSON is readable and contains findings, duplicate sets, totals, and files scanned.
• Export errors show a friendly message.