V2 Make FileId type opaque and use consistently throughout project (#4307)

# Description of Changes
The `FileId` type in V2 currently is just defined to be a string. This
makes it really easy to accidentally pass strings into things accepting
file IDs (such as file names). This PR makes the `FileId` type [an
opaque
type](https://www.geeksforgeeks.org/typescript/opaque-types-in-typescript/),
so it is compatible with things accepting strings (arguably not ideal
for this...) but strings are not compatible with it without explicit
conversion.

The PR also includes changes to use `FileId` consistently throughout the
project (everywhere I could find uses of `fileId: string`), so that we
have the maximum benefit from the type safety.

> [!note]
> I've marked quite a few things as `FIX ME` where we're passing names
in as IDs. If that is intended behaviour, I'm happy to remove the fix me
and insert a cast instead, but they probably need comments explaining
why we're using a file name as an ID.

frontend/src/hooks/useFileHandler.ts

@@ -1,6 +1,7 @@
 import { useCallback } from 'react';
 import { useFileState, useFileActions } from '../contexts/FileContext';
 import { FileMetadata } from '../types/file';
+import { FileId } from '../types/file';
 
 export const useFileHandler = () => {
   const { state } = useFileState(); // Still needed for addStoredFiles
@@ -17,16 +18,16 @@ export const useFileHandler = () => {
   }, [actions.addFiles]);
 
   // Add stored files preserving their original IDs to prevent session duplicates
-  const addStoredFiles = useCallback(async (filesWithMetadata: Array<{ file: File; originalId: string; metadata: FileMetadata }>) => {
+  const addStoredFiles = useCallback(async (filesWithMetadata: Array<{ file: File; originalId: FileId; metadata: FileMetadata }>) => {
     // Filter out files that already exist with the same ID (exact match)
     const newFiles = filesWithMetadata.filter(({ originalId }) => {
       return state.files.byId[originalId] === undefined;
     });
-    
+
     if (newFiles.length > 0) {
       await actions.addStoredFiles(newFiles);
     }
-    
+
     console.log(`📁 Added ${newFiles.length} stored files (${filesWithMetadata.length - newFiles.length} skipped as duplicates)`);
   }, [state.files.byId, actions.addStoredFiles]);
 
@@ -35,4 +36,4 @@ export const useFileHandler = () => {
     addMultipleFiles,
     addStoredFiles,
   };
-};
+};