Preserve local paths for desktop saves (#5543)

# Summary

- Adds desktop file tracking: local paths are preserved and save buttons
now work as expcted (doing Save/Save As as appropriate)
- Adds logic to track whether files are 'dirty' (they've been modified
by some tool, and not saved to disk yet).
- Improves file state UX (dirty vs saved) and close warnings
- Web behaviour should be unaffected by these changes

## Indicators
Files now have indicators in desktop mode to tell you their state.

### File up-to-date with disk

<img width="318" height="393" alt="image"
src="https://github.com/user-attachments/assets/06325f9a-afd7-4c2f-8a5b-6d11e3093115"
/>

### File modified by a tool but not saved to disk yet

<img width="357" height="385" alt="image"
src="https://github.com/user-attachments/assets/1a7716d9-c6f7-4d13-be0d-c1de6493954b"
/>

### File not tracked on disk

<img width="312" height="379" alt="image"
src="https://github.com/user-attachments/assets/9cffe300-bd9a-4e19-97c7-9b98bebefacc"
/>

# Limitations
- It's a bit weird that we still have files stored in indexeddb in the
app, which are still loadable. We might want to change this behaviour in
the future
- Viewer's Save doesn't persist to disk. I've left that out here because
it'd need a lot of testing to make sure the logic's right with making
sure you can leave the Viewer with applying the changes to the PDF
_without_ saving to disk
- There's no current way to do Save As on a file that has already been
persisted to disk - it's only ever Save. Similarly, there's no way to
duplicate a file.

---------

Co-authored-by: James Brunton <[email protected]>
Co-authored-by: James Brunton <[email protected]>

frontend/src-tauri/src/commands/files.rs

@@ -33,3 +33,12 @@ pub async fn clear_opened_files() -> Result<(), String> {
     Ok(())
 }
 
+// Command to atomically get and clear opened file paths
+#[tauri::command]
+pub async fn pop_opened_files() -> Result<Vec<String>, String> {
+    let mut opened_files = OPENED_FILES.lock().unwrap();
+    let all_files = opened_files.clone();
+    opened_files.clear();
+    add_log(format!("📂 Returning and clearing {} opened file(s)", all_files.len()));
+    Ok(all_files)
+}

frontend/src-tauri/src/commands/mod.rs

@@ -5,7 +5,7 @@ pub mod auth;
 pub mod default_app;
 
 pub use backend::{cleanup_backend, get_backend_port, start_backend};
-pub use files::{add_opened_file, clear_opened_files, get_opened_files};
+pub use files::{add_opened_file, clear_opened_files, get_opened_files, pop_opened_files};
 pub use connection::{
     get_connection_config,
     is_first_launch,