Editor: no hover tooltips for symbols, types, or diagnostics

[KrisPowers]

Hovering over a symbol, type, or diagnostic marker in the editor shows nothing. There are no hover cards, no type information on hover, and no way to read the full text of a diagnostic error without switching to a panel.

This is a significant usability gap, especially for diagnostics. When a red gutter bar appears on a line, there is no way to see what the error is without looking at a separate diagnostics panel.

What needs to happen

At minimum, diagnostic hover should be implemented first since the diagnostic data is already available:

• When the user hovers over a line that has a diagnostic (red or yellow gutter bar, or an underlined region), show a small tooltip popup with the full diagnostic message, source, and error code.
• The tooltip should appear near the cursor after a short delay (~400ms) and dismiss when the cursor moves away.

As a stretch goal, symbol hover could show:

• The inferred type of the symbol (if LSP or other type info is available)
• The JSDoc/docstring associated with a function or class

Files likely involved

• app/frontend/src/components/GpuEditor.tsx -- mouse move handler, tooltip state and rendering
• Diagnostic data is already passed to the editor; it needs to be queryable by line number on hover

Acceptance criteria

• Hovering over a line with a diagnostic shows a tooltip with the error/warning message
• The tooltip respects the editor's theme (dark/light)
• The tooltip does not flicker or reposition erratically
• It dismisses cleanly when the mouse moves to a different area