Evals UI: Fix inconsistent terminology

[nishika26]

Describe the current behaviour

The text evaluation UI uses inconsistent terminology across different components:

• Tabs and navigation refer to "Upload" and "Results"
• Some button labels and guidance text still use the old naming convention
• Tab initialization defaults to the outdated terminology

This inconsistency exists because terminology was updated from "upload" → "datasets" and "results" → "evaluations" during initial development, but some references were missed in the implementation.

Describe the enhancement you'd like

Update all remaining UI text to use consistent terminology throughout the text evaluation feature.Specific changes needed:

• Rename all instances of "Upload" to "Datasets"
• Rename all instances of "Results" to "Evaluations"
• Update tab labels, navigation targets, and button text
• Update guidance text and helper messages
• Modify default tab initialization to use "Datasets"

This ensures complete naming consistency across:

• Tab headers
• Navigation links
• Button labels
• User guidance text
• Default state initialization