Fix layout shifts after selecting custom Blueprint

[bcotrim]

Related issues

• Fixes STU-1219

Proposed Changes

• Add fixed height (h-7) to the HStack containing "Featured Blueprints" text and the Blueprint file selector
• This prevents layout shifts when toggling between the "Choose Blueprint file" button and the selected file name with "Remove" button

Testing Instructions

1. Start Studio
2. Click "Add site" → "Start from a Blueprint"
3. Click "Choose Blueprint file" and select a valid Blueprint JSON file
4. Verify no layout shift occurs when the file is selected
5. Click "Remove" to clear the selection
6. Verify no layout shift occurs when returning to the upload button state
7. Repeat selection/removal a few times to confirm stability

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[epeicher]

Thanks @bcotrim! I have tested it, and I can confirm that I don't see the layout shift anymore when adding or removing blueprints. LGTM!

[katinthehatsite]

This looks good to me, I did not notice any more layout shifts.

[wpmobilebot]

📊 Performance Test Results

Comparing daaa39b vs trunk

site-editor

Metric	trunk	daaa39b	Diff	Change
load	2858.00 ms	2864.00 ms	+6.00 ms	🔴 0.2%

site-startup

Metric	trunk	daaa39b	Diff	Change
siteCreation	8079.00 ms	7090.00 ms	-989.00 ms	🟢 -12.2%
siteStartup	3943.00 ms	3940.00 ms	-3.00 ms	🟢 -0.1%

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change