fix(ui): align the window controls with the title bar

[mrsibe]

What does this PR do?

Fixes the title-bar / window-control geometry mismatch on Windows and Linux, makes a small pass over the theme and layout details around it, and corrects DESIGN.md where it describes something the code no longer does.

Why?

The OS draws minimize / maximize / close into a region whose height is
titleBarOverlay.height. It was 35px while the renderer's title bar was
h-11 (44px), so on Windows the controls sat above the centre of the bar they
appear to belong to.

Fixing that exposed three neighbours in the same surface, so they are fixed here
rather than left as follow-ups:

• The renderer reserved w-32 (128px) for the controls, but three caption
  buttons occupy 138px — 10px short, which let the settings button slide under
  the minimize button.
• Linux reserved nothing at all, so its tab strip ran underneath the controls.
• surface-hover is translucent, so it cannot give an opaque surface-raised
  card a visible hover in dark mode: surface-hover composited over
  surface-base lands on almost the same lightness as surface-raised. That is
  why the card hover is composited as an overlay rather than swapped in.

Related issue

Related to #26 (same titleBarOverlay module; that PR fixed the symbol colour,
this one fixes the geometry).

What changed?

Window chrome

• New src/shared/utils/windowChrome.ts holds TITLE_BAR_HEIGHT (44px) and
  WINDOW_CONTROLS_WIDTH (138px). The main process and the renderer both read
  from it, so the two halves of the title bar cannot drift apart again.
• titleBarOverlayOptions() now returns height: TITLE_BAR_HEIGHT (was a
  literal 35).
• TopNavigationBar reserves the control strip on every non-macOS platform, at
  the real caption-button width.
• test/titleBarOverlay.test.ts asserts the overlay height equals the renderer
  bar height and that the strip is 3 × 46px, so a regression fails a test rather
  than showing up on someone's Windows machine.

Theme

• theme.css declares color-scheme per theme. Native surfaces the CSS does not
  paint — scrollbars, caret, text selection, form controls, <select> popups,
  autofill — were rendering in light mode inside the dark theme.
• Document title was still Electron.
• The boot screen's hardcoded Loading... now uses the existing
  common.loading key and a spinner, so it is translated and not a bare string.

Layout

• Home page gutters: the hero used px-6 and the notebook grid px-12, so the
  page title and the first card did not share a left edge. Both now use px-6
  (the page gutter in DESIGN.md), as does the empty state.
• NotebookCard no longer renders a --chart-* colour rail. DESIGN.md forbids
  chart colours outside chart/graph surfaces, and the rail was the only place
  they appeared as decoration. The card picks up the hover state it was missing.
• Blockquotes are 2px in both reading surfaces; they were 4px in chat and 3px in
  the editor.

Documentation (docs(design) commit)

impeccable doctor reports one finding (design-md-coverage), but reading
DESIGN.md against the code found several places where the document describes
something the code does not do. No rendered pixel changes.

• Form field recipe was wrong twice. It said the label is text-xs and the
  control h-8; Label/FieldLabel are text-sm font-medium, Input and the
  Select trigger are h-9, and the density table already said h-9 — the
  recipe contradicted both the components and the table. Error text was
  documented as text-xs; FieldError is text-sm.
• components/ui/field.tsx was never documented, yet it is what the settings
  surfaces use. The recipe described a hand-rolled stack instead.
• The title-bar density tier was undocumented (28px tabs and icon buttons in
  a 44px bar), which made TopNavigationBar read as a violation of the size-8
  toolbar-icon-button rule rather than a deliberate tier.
• Tabs was documented as having exactly one form. The open-notebook strip is
  a second form, and it expresses selection with the translucent fill rather than
  a segmented control's surface step.
• The border-none opt-out that fix(ui): remove the dashed border from the knowledge base empty state #58 added to panel-level empty states was not
  recorded.
• The legacy-alias list was incomplete, and implied --secondary was one of those
  aliases. It is not: it holds its own literals, nothing renders it (Button and
  Badge variant="secondary" use bg-muted), and its light value is the only
  blue-tinted neutral in the palette. DESIGN.md now marks it for deletion rather
  than adoption.

How was this tested?

• npm run typecheck — passes.
• npm run lint — 0 errors (113 pre-existing warnings, unchanged).
• npm test — 50/50 pass, including the new geometry assertions.
• npm run check:design — no violations.
• npx electron-vite build — main, preload and renderer all build; verified in
  the output that main carries TITLE_BAR_HEIGHT = 44 and the renderer carries
  WINDOW_CONTROLS_WIDTH = 138 used by the title-bar spacer.
• npx prettier --check — clean.

Not verified: the actual on-screen result of the controls against the bar. That
needs a Windows (and Linux) run — I could not capture it from this environment,
and I am not going to publish screenshots of the local notebook data that a
running instance would show. The geometry is asserted by test instead; a
screenshot from a Windows build would still be worth having before merge.

Open decisions this PR does not make

Both were found while checking DESIGN.md and need a call rather than a doc edit:

1. The surface table claims the title bar is surface-sunken; it renders
   surface-base. TopNavigationBar carries no background class and inherits
   the page root, so the bar is the same tone as the canvas, not a distinct chrome
   band. Either the table row is wrong, or the bar should get bg-surface-sunken
   — that second option is a visible change, so I left it alone.
2. --secondary / --secondary-foreground are dead. Now documented as such
   in DESIGN.md; deleting them from theme.css is a separate, mechanical change.

Screenshots / recordings

No before/after captures attached (see above). Happy to add them if someone can
run a Windows build.

Checklist

• I have reviewed my own changes.
• npm run typecheck passes.
• npm run build passes.
• I have tested the affected user workflow. (Windows/Linux title bar not run)
• I have not included unrelated changes.
• I have updated documentation when necessary.

Desktop / build changes

• Not applicable