Stop Train tab from running dataset prep on open

[Legendarylibr]

Summary

• Train no longer pre-fills the sample dataset or auto-picks the first local model when the sidebar tab is opened.
• Full-corpus dataset analysis and related prep only run after the user selects a dataset (or model for lighter recommendations).
• Adds a short UI hint that nothing runs until a dataset is chosen.

Test plan

• Open Train from the sidebar with no prior selection — no “Analyzing dataset…” banner and no analyze-dataset/recommendations traffic until a choice is made.
• Select the bundled sample (or any HF/local path) — analysis starts and suggested settings appear.
• Clear dataset / re-open tab — analysis does not re-run on empty selection.
• Hub deep-link with ?model= still preselects/downloads as before.