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/components/shared/FileGrid.tsx

@@ -5,6 +5,7 @@ import SearchIcon from "@mui/icons-material/Search";
 import SortIcon from "@mui/icons-material/Sort";
 import FileCard from "./FileCard";
 import { FileRecord } from "../../types/fileContext";
+import { FileId } from "../../types/file";
 
 interface FileGridProps {
   files: Array<{ file: File; record?: FileRecord }>;
@@ -12,8 +13,8 @@ interface FileGridProps {
   onDoubleClick?: (item: { file: File; record?: FileRecord }) => void;
   onView?: (item: { file: File; record?: FileRecord }) => void;
   onEdit?: (item: { file: File; record?: FileRecord }) => void;
-  onSelect?: (fileId: string) => void;
-  selectedFiles?: string[];
+  onSelect?: (fileId: FileId) => void;
+  selectedFiles?: FileId[];
   showSearch?: boolean;
   showSort?: boolean;
   maxDisplay?: number; // If set, shows only this many files with "Show All" option
@@ -119,11 +120,11 @@ const FileGrid = ({
         direction="row"
         wrap="wrap"
         gap="md"
-        h="30rem" 
+        h="30rem"
         style={{ overflowY: "auto", width: "100%" }}
       >
         {displayFiles.map((item, idx) => {
-          const fileId = item.record?.id || item.file.name;
+          const fileId = item.record?.id || item.file.name as FileId /* FIX ME: This doesn't seem right */;
           const originalIdx = files.findIndex(f => (f.record?.id || f.file.name) === fileId);
           const supported = isFileSupported ? isFileSupported(item.file.name) : true;
           return (