feat: hover previews and draggable pinned chart callouts

[isConic]

Hovering an inspectable map asset now shows a compact chart preview. Clicking pins it; several pinned charts can be dragged by their headers while thin connectors track the original map locations during pan and zoom. Background clicks preserve pins, and Close or Escape dismisses them.

The callout shows one asset title and its attachments, removing the description panel, repeated attachment headings, expansion button, and redundant links. HTML stays in the existing opaque sandbox; previews are inert until pinned. This remains a generic metadata-driven viewer feature, with no station or pixel-drill asset type.

Validation:

• 20 browser, embed-mode, and asset-cache tests pass, including real HTTP-cache upgrade coverage.
• Read-only browser verification against a saved Lake Cawndilla map with local code overrides passed hover, pin, drag, multiple charts, moving connectors, plot interaction, and iframe Escape.
• JavaScript syntax and diff checks pass.

Companion EOGPT #173 supplies compact Plotly attachments and distinguishes outside clicks from Escape in the embedding page.