Fix async save flow in firestore upload and download (closes #38)

[Prachi01Yadav]

Fix: Async save flow bugs in firestore.ts (Issue #38)

What was wrong

Bug 1: Double save in uploadFileToCloud:
When a file with a duplicate name was detected, the code correctly
prompted for a new name and saved the renamed file. But there was no
return` statement after that save. So the function kept running and
saved the file a second time under the original duplicate name,
creating two copies in Firestore.

Bug 2 : Missing await in downloadFileFromFirebase:
local._saveFile() is an async function but was called without await
in two places. This means the file save was started but not finished
before the success callback fired : causing silent save failures where
the user sees "File Downloaded" but the file was not actually saved.

Bonus fix : Variable redeclaration:
The variable prevFile was declared twice using const in the same
scope inside uploadFileToCloud, which is a JavaScript error.
Renamed the inner one to prevFileWithNewName to fix this.

What I changed

• src/firebase/firestore.ts : 3 fixes in this one file

How to test

1. Upload a file, then upload another file with the same name
   → Before fix: two copies saved. After fix: only the renamed copy saved.
2. Download a file from Firebase
   → Before fix: "Downloaded" shown but file not saved. After fix: file
   properly saved before callback fires.

Fix: Async bugs in Firebase cloud save operations (firestore.ts)

Related to the async reliability theme of #38, but fixes separate
bugs specifically in the Firebase cloud save layer:

1. Double-save bug in uploadFileToCloud — missing return after
   rename caused file to be saved twice
2. Missing await in downloadFileFromFirebase — _saveFile() called
   without await, causing silent failures
3. Variable redeclaration error (const prevFile declared twice)

Note: The local save flow bugs in Menu.tsx and NewFile.tsx are
being addressed separately in PR #39.