refactor: drop automatic bandwidth calculations and normalize deprecated AutoLimit

[Rello]

Motivation

• Remove legacy automatic/relative bandwidth throttling logic which is no longer required and simplifies bandwidth handling to absolute limits only.
• Ensure old/deprecated settings (AutoLimit and LegacyGlobalLimit) are normalized to NoLimit during load and when applied so upgrades don't reintroduce throttling.
• Stop applying a default 75% automatic throttle and default to unlimited unless a manual limit is set.

Description

• Remove relative/automatic bandwidth measurement and quota logic from BandwidthManager and simplify its interface/state to handle only absolute upload/download limits (changes in src/libsync/bandwidthmanager.cpp and src/libsync/bandwidthmanager.h).
• Normalize deprecated auto/legacy settings to unlimited in account handling by persisting normalized values on load and mapping AutoLimit/LegacyGlobalLimit to NoLimit in setters (changes in src/gui/accountmanager.cpp, src/libsync/account.cpp, src/libsync/account.h).
• Remove the automatic option from the network settings UI and update loadBWLimitSettings()/saveBWLimitSettings() to only expose ManualLimit and NoLimit (changes in src/gui/networksettings.cpp and src/gui/networksettings.ui).
• Stop defaulting to a 75% automatic throttle in the sync engine by defaulting network limits to unlimited (0) in Folder::setDirtyNetworkLimits() (change in src/gui/folder.cpp).

Testing

• No automated tests were run for this change.

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Codex Task

[mgallien]

one small comment

[mgallien]

if we are going to trust the comment, this timer should possibly be removed also

[github-actions]

Artifact containing the AppImage: nextcloud-appimage-pr-9400.zip

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