fix(#470): render a Dashboard filter diagnostic in place, not a second row

[BorisTyshkevich]

What & why

Closes #470.

A Dashboard filter's "no inferred control" diagnostic (shown for
container-typed variables — Array/Tuple/Map/Nested — with no option SQL
configured, e.g. Array(Int32)) used to render as a second element
appended beside the input. Since that element and the input's own wrapper were
both pinned to the same CSS grid column (.var-field's 2-column grid), grid
auto-placement pushed it into a visible second row — reading as a duplicate
control repeating the declared type text, rather than an error/warning state
on the existing filter.

The diagnostic now adorns the same .var-input in place, mirroring the
existing is-invalid/is-conflict affordances already established in
var-field.ts:

• a new is-unsupported class carries the yellow/dashed warning styling on
  the input itself;
• a decorative warning icon is absolutely positioned over the input (the same
  technique the inline clear (×) button already uses), so it can never add a
  new grid row;
• the full diagnostic message reaches pointer hover via title — folded into
  baseTitle itself (not assigned once after build) so it survives every
  later keystroke/commit, which reset title from baseTitle via
  applyFieldState and would otherwise silently revert to the plain type
  tooltip the moment the user typed;
• the full diagnostic also reaches keyboard focus/screen readers via
  aria-describedby, pointing at a visually-hidden .sr-only sibling.

The input's rendered width is unchanged (verified pixel-identical, 102.3px,
between diagnostic and non-diagnostic fields in a real browser check) — the
extra padding-right for the icon eats into the box via the project's global
box-sizing: border-box, it doesn't grow it.

Verified visually in a real browser (grid layout is invisible to the
happy-dom unit suite): before the fix, the marker lands in its own row below
the input; after the fix, one row, aligned with any other filter in the same
strip.

Checklist

• npm test passes (the per-file coverage gate is non-negotiable)
• Tests added/updated in the same change as the code
• npm run build succeeds (single-file dist/sql.html)
• Layers kept honest: pure logic in src/core/, network in src/net/ (injected fetch), DOM in src/ui/
• No new runtime dependency
• CHANGELOG.md ([Unreleased]) updated
• No tracked roadmap work reshaped (standalone bug fix, not part of a phased issue)