docs(design): correct DESIGN.md against the current tokens and components

[mrsibe]

What does this PR do?

Corrects DESIGN.md where it describes something the code does not do, or leaves out a value the code already uses. Documentation only — no rendered pixel changes.

Follow-up to the merged #84: that PR added the Window chrome section and the Raised card recipe, and this pass is the rest of the same check.

Why?

impeccable doctor reports one finding (design-md-coverage), but that finding is about heading names, not content. Reading DESIGN.md against the current tokens and components turns up several places where the document is simply wrong about the code, and a few values the code uses that the document never states.

Because DESIGN.md is the source of truth for UI decisions, a wrong recipe is worse than a missing one: the next agent copies it.

What changed?

Wrong about existing code

• The Form field recipe contradicted both the components and the density table.
  It said the label is text-xs and the control h-8:

  • Label is text-sm font-medium leading-none; FieldLabel and the
    hand-written labels in ModelConnectionForm are text-sm font-medium.
  • Input is h-9 and the Select trigger is h-9, and the density table
    already said Input / Select trigger | h-9. The recipe's h-8 is the sm
    button height and the compact row height, not an input height.
  • Error text was documented as text-xs; FieldError is
    text-destructive text-sm font-normal with role="alert".

• components/ui/field.tsx was never documented, yet it is what the settings
  surfaces use (GeneralSettings renders FieldLabel). The recipe described a
  hand-rolled label/input/error stack. The primitives are now named, and the
  recipe points at FieldError instead of restating its classes.

Missing values the code already uses

• The title-bar density tier. TopNavigationBar runs 28px tabs and 28px icon
  buttons inside the 44px bar. Without a row in the density table, that reads as a
  violation of Toolbar icon button | size-8 rather than a deliberate tier, so
  the tier is now documented as stopping at the title bar.
• Tabs was documented as having exactly one form — the segmented control,
  where "selection is expressed as a surface step". The open-notebook strip in the
  title bar is a second, legitimate form: no track, and selection expressed with
  the ordinary translucent bg-surface-selected fill. Both are now written down.
• The border-none opt-out that fix(ui): remove the dashed border from the knowledge base empty state #58 added to the panel-level empty states was
  never recorded, so the next reader would reasonably reintroduce the dashed
  outline in the knowledge-base panel.

Smaller corrections

• The legacy-alias list was incomplete, and it implied --secondary was one of
  those aliases. It is not: it holds its own literals in both themes, nothing
  renders it (Button and Badge variant="secondary" both use bg-muted), and
  its light value is the only blue-tinted neutral in the palette. DESIGN.md now
  says so and marks it for deletion rather than adoption.

Open decisions this PR does not make

Found by the same pass; both need a call rather than a doc edit, so neither is in
this diff:

1. The surface table claims the title bar is surface-sunken; it renders
   surface-base. TopNavigationBar carries no background class, so it
   inherits the page root and is the same tone as the canvas rather than a
   distinct chrome band. Either the table row is wrong, or the bar should get
   bg-surface-sunken. The second option is a visible change.
   (Everything else that row names — the settings Sidebar, the settings dialog
   shell — does use bg-surface-sunken.)
2. --secondary / --secondary-foreground are dead tokens. Now documented as
   such; deleting them from theme.css is a separate mechanical change.

How was this tested?

• npx prettier --check DESIGN.md — clean.
• Every claim above was read out of the source it names before it was written:
  ui/label.tsx, ui/input.tsx, ui/select.tsx, ui/field.tsx,
  ui/tabs.tsx, common/TopNavigationBar.tsx, settings/ModelConnectionForm.tsx,
  notebook/source/DocumentList.tsx, assets/theme.css.
• npm run check:design — no violations.

No code changed, so npm test / npm run build were not re-run.

Screenshots / recordings

Not applicable — documentation only.

Checklist

• I have reviewed my own changes.
• npm run typecheck passes. (unchanged; no code touched)
• npm run build passes. (unchanged; no code touched)
• I have tested the affected user workflow. (n/a)
• I have not included unrelated changes.
• I have updated documentation when necessary.

Desktop / build changes

• Not applicable