fix: fullscreen overlay issues with header, map, and controls (#887, #859, #829)

[NewCoder3294]

Summary

• Cannot change between World/Tech/Financial in Fullscreen (Chrome) #887: Header now gets z-index: 10001 when a panel is fullscreen, keeping the variant switcher (World/Tech/Financial) clickable
• Full screen webcam is overlayed by new bottom panel #859: Map section is hidden when a panel goes fullscreen, preventing it from showing behind the fullscreen panel
• Map legend and filters on top layer #829: Map legend and layer toggle controls are hidden during panel fullscreen via display: none

Test Plan

• Enter panel fullscreen (e.g., Live News or Webcams) → variant switcher in header is clickable
• Enter webcam fullscreen → no map content visible behind it
• Enter any panel fullscreen → map legend and layer toggles are not visible
• Exit fullscreen → everything returns to normal

🤖 Generated with Claude Code

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[koala73]

Superseded by #1313 which takes a better approach: hiding the entire .map-section container with a single rule instead of individual elements. That PR has been rebased onto main and is ready for review.