Check POST request status before displaying success

[Gold856]

Description

We currently don't check if the POST request succeeded before immediately sprinting off to reload the page. This often means users see a green "reloading" notification, which I think is masking issues in https://www.chiefdelphi.com/t/photonvision-offline-updating-to-2026-1-1-not-working/512677. Confirmed that a disconnected UI (the demo page) will always show that green reloading notification. Fix this by making axiosPost return whether or not the POST request succeeded, and only refresh if it succeeded. This also similarly pulls out the snackbar message displayed on upload completion out of the upload progress callback and into an if statement because it's cleaner and avoids potential rounding errors causing it to not display.

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Meta

Merge checklist:

• Pull Request title is short, imperative summary of proposed changes
• The description documents the what and why, including events that led to this PR
• If this PR changes behavior or adds a feature, user documentation is updated
• If this PR touches photon-serde, all messages have been regenerated and hashes have not changed unexpectedly
• If this PR touches configuration, this is backwards compatible with all settings going back to the previous seasons's last release (seasons end after champs ends)
• If this PR touches pipeline settings or anything related to data exchange, the frontend typing is updated
• If this PR addresses a bug, a regression test for it is added