fix: write network for db same as for cluster (utilization - doughnut, throughput - number)

[Copilot]

This PR fixes the inconsistency between cluster and database network metrics display formats.

Problem:
Database network metrics were showing throughput in "X of Y" format (e.g., "5.2 MB/s of 10.5 MB/s") while cluster network metrics show just the throughput value (e.g., "5.2 MB/s"). This inconsistency made the UI confusing for users switching between cluster and database views.

Solution:
Updated the database network metrics in DedicatedMetricsTabs.tsx to use the same formatter as cluster metrics:

• Before: Used formatSpeedLegend({value, capacity}) showing "X of Y" format
• After: Created formatNetworkThroughputLegend({value}) using formatBytes({value, withSpeedLabel: true}) - same as cluster

Result:
Both cluster and database now consistently display:

• Utilization: Doughnut chart with percentage (unchanged, was already correct)
• Throughput: Simple number with speed label (e.g., "5.2 MB/s")

The change is minimal and surgical - only affects the network throughput legend formatter while preserving all existing functionality including the doughnut chart visualization for utilization.

Fixes #2880.

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