Add street view / walkthrough mode

[Yashism]

Hey! This adds a street view mode to the editor — basically an FPS-style walkthrough so you can explore your home designs from the inside.

What it does

• WASD to walk around, mouse to look (pointer lock)
• Q/E to float up and down
• Shift to sprint
• Camera starts at the center of the grid at eye height (1.65m)
• Can't go below ground level
• No collision — you can walk through walls freely
• ESC or the exit button to go back to the editor

Where to find it

There's a new walk icon button in the camera actions bar at the bottom. Also added one in the preview mode overlay.

What changes

• New FirstPersonControls component that handles movement + mouse look
• useEditor gets an isFirstPersonMode flag
• Orbit controls are disabled while in street view
• Editor UI (tools, panels, sidebar, grid) is hidden during walkthrough
• Walls are forced to full height when entering street view