Code Preview for linked maps

# Feature request

In #686 we linked cc.jsons to their source code. But that requires leaving CodeCharta and opening a different tool. What if we just want a quick look at the source code and then continue?

## Description

**Given** a map has been linked to its source code
**and** my mouse hovers over a building
**When** I press space on my keyboard
**Then** a modal opens that shows me the source code

**Given** a map has been linked to its source code
**and** I have selected a building
**When** I press space on my keyboard
**Then** a modal opens that shows me the source code

**Given** a map has been linked to its source code
**and** I have selected a building
**When** I look at the file attribute bar
**Then** I can see a clickable Preview button directly under the url attribute

**Given** a map has NOT been linked to its source code
**and** my mouse hovers over a building
**When** I press space on my keyboard
**Then** a message informs me that I should link the source code

## Tech notes

The preview window should use some JavaScript library that can syntax hightlight a lot of programming languages.

Start by reviewing issue #686 and the existing linked source-code behavior. Trace how a selected or hovered building exposes its file attributes, then determine where the Preview button, space-key handling, and modal belong. Done means linked maps show the source preview in a modal, while unlinked maps display the requested guidance message.

Written by the indexing model from the issue text.