Add PDF archive store, raw-body IPC and export file helpers

The searchable PDF can be archived content-addressed under the local data
dir with its sha256 and layout fingerprint; a different PDF for an archived
invoice is refused and reads verify the hash. Export writes go through a
raw-body command that remembers the last folder; reveal-in-folder and open
fall back to xdg-open / explorer. Adds a Windows-safe export file name
builder. Migration M4 adds the archive columns and last export dir.

src-tauri/src/commands/assets.rs

@@ -133,22 +133,6 @@ pub fn remove_asset(state: State<AppState>, path: String) -> Result<(), String>
     remove_asset_file(&state.data_dir, &path)
 }
 
-/// Write a base64 payload (e.g. a generated PDF) to a user-chosen path.
-#[tauri::command]
-pub fn save_binary_file(path: String, data_base64: String) -> Result<(), String> {
-    if path.trim().is_empty() {
-        return Err("No file path given".into());
-    }
-    let bytes = STANDARD
-        .decode(data_base64.as_bytes())
-        .map_err(|e| e.to_string())?;
-    if let Some(parent) = Path::new(&path).parent() {
-        // A missing parent is created; if that fails the write below reports the real error.
-        std::fs::create_dir_all(parent).ok();
-    }
-    std::fs::write(&path, bytes).map_err(|e| e.to_string())
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -248,9 +232,4 @@ mod tests {
         assert!(remove_asset_file(dir.path(), "voiced.db").is_err());
         assert!(dir.path().join("voiced.db").exists());
     }
-
-    #[test]
-    fn save_binary_file_rejects_empty_path() {
-        assert!(save_binary_file("  ".into(), "AAAA".into()).is_err());
-    }
 }