feat(ui): move layout tabs & widget picker to drop zone

[NewCoder3294]

Summary

• Move layout tabs (Intelligence, Markets, News, Minimal) and widget picker (+) button from the header into the map bottom drop zone
• Add dashed-border drop zone with centered "DROP PANELS HERE TO MOVE THEM BELOW THE MAP" label
• Panels dropped into the zone use a 2-column grid layout
• Label auto-hides when panels are present
• Header is now cleaner, matching production

Files changed

• src/app/panel-layout.ts — moved mount points from header to drop zone
• src/styles/main.css — drop zone controls styling, grid layout, dashed border label

Test plan

• Layout tabs and + button appear at top of drop zone below the map
• Header no longer contains layout tabs or widget picker
• Clicking layout tabs still switches presets
• Widget picker (+) still opens popover
• Drag-and-drop panels into the zone works, panels show in 2-column grid
• Drop zone label hides when panels are present
• Responsive: controls hidden on mobile (< 900px)
• npm run typecheck passes

🤖 Generated with Claude Code

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[NewCoder3294]

This is an update to Issue #882

Here are some images. It works similarly for tech and finance. It also works for light mode. It will first pop up in the right panel, and then the user can drag and drop it to the left panel.