Show merged storage in a dedicated tab

[alvins82]

What changed

• adds a dedicated Merged Storage tab to Storage Manager
• keeps the Merge storages action inside that tab whether or not a pool is configured
• keeps every filesystem visible under Storage and marks mergerfs member disks as Merged
• presents /DATA as one CasaOS HD pool summary with its source count and aggregate capacity
• shows the mandatory system source in the merge dialog as checked and disabled, using the public /DATA path
• resolves the backing system filesystem from the actual base source path while excluding unrelated partitions such as /boot
• hides Format and Remove actions for active mergerfs member disks
• opens the merged-storage tab by default when a CasaOS HD pool exists
• returns to the merged-storage tab after a successful merge

Why

A successful merge previously appeared to do nothing. The first tab implementation then treated every system partition as a pool source and emptied the Storage tab. The merge dialog also hid the mandatory system source, making its three-source capacity confusing. This revision represents the mergerfs pool separately while preserving an accurate view of the underlying filesystems and all included sources.

Validation

• pnpm run build
• deployed the production build to the Ubuntu 26 storage-test VM at 192.168.252.8
• verified an empty Merged Storage tab displays the Merge storages button
• verified that empty-state button opens the merge dialog
• verified the merge dialog displays system storage as Always included · /DATA with a checked and disabled checkbox
• verified external storage checkboxes remain selectable
• verified /boot is excluded from pool capacity
• verified the Storage tab no longer contains the merge action or an empty footer