feat(firestore): allow users to overwrite existing cloud files on upload

[vibhutomer]

Fix: Enable Cloud File Overwrites / Updates

Related Issue

• Closes Cannot Overwrite/Update Existing Cloud Files (Forced Rename) #75

Description of Changes

• Refactored the uploadFileToCloud function in firestore.ts.
• Previously, the app forced a rename operation if a file already existed in the cloud, removing the ability to "update" or "sync" existing invoices.
• Implemented a confirm() dialog. If the user chooses to overwrite, the code seamlessly updates the existing document. If they decline, it prompts them to save as a new copy.

Impact

• Core Functionality Restored: Users can now actually save changes to their existing cloud invoices without generating duplicates.
• Improved UX: Replaced the jarring alert() with a clear, standard "Do you want to overwrite?" workflow.
• Cleaner Database: Prevents users from cluttering their Firebase storage with unnecessary versioned files (invoice_1, invoice_2, etc.).