feat(appearance): add Calm/Signature visual style, readability mode, and chart palette (#1407)

* feat(appearance): add Calm/Signature visual style, readability mode, and chart palette

Turn the "too intense / italic headers hurt / the security graph fights my
eyes" feedback into a token-driven Visual style with Calm as the new default
and Signature one click back to the prior look.

- Heading family routes through a `.font-heading` utility driven by
  `--font-heading`/`--heading-style`: operational headings render upright in the
  interface face under Calm and italic Instrument Serif under Signature. Base
  rule sets family + style only, so each call site keeps its own weight/tracking
  and Signature stays a true no-op; the Calm lift is a `[data-headings="clean"]`
  descendant rule. Brand lockup, empty-state heroes, and onboarding stay serif.
- Severity charts resolve through `--sev-*` tokens with Muted, Heat, and
  Signature palettes; FindingsByType routes its series through the severity ramp
  plus a neutral so no brand-cyan sits next to rose. The risk trend flattens its
  gradient under Muted/Heat/reduced and keeps the gradient under Signature.
- Appearance settings gain Visual style cards, a Security visualization palette,
  a Readability master toggle, a Motion & effects group, and a "Reset to default"
  button (restores the Calm axes, disabled while readability is on). Contrast
  moves under Readability and Ambient glow under Motion & effects. A card is
  selected only while the stored sub-axes match its preset, so a custom
  combination de-selects both.
- The topbar Theme quick-switch swaps the interface/data font pickers for a
  Visual style switch and a Readability toggle (text size kept); its footer
  Settings link jumps straight to Appearance.
- Readability is a sticky master that forces the calm resolution and a contrast
  lift at apply time without mutating the stored sub-axes.
- New users default to Calm; any pre-existing persisted appearance state keeps
  the Signature look. The pre-paint script mirrors the store.
- SegmentedControl gains a `disabled` prop and a nullable value (no active
  segment for a custom combination, with a roving-tabindex keyboard anchor).
  Adds unit/component coverage for the store, migration, chart shape logic, the
  disabled control, the reset/de-selection, and the quick-switch.

* fix(appearance): migrate Blueprint serif headings and surface readability locks

- Migrate the two operational Blueprint headings (catalog tile name, drift-policy
  option title) from font-serif italic to the .font-heading utility; the first
  pass only covered font-display, so Calm still left these italic. font-serif and
  font-display both resolve to the same display face, so this is the same fix.
- Lock the Visual style cards under Readability (parity with the topbar switch and
  the on-screen guidance to turn Readability off to choose a style by hand).
- Lock the Border brightness slider under Readability and show its forced +0.03
  readout, since Readability overrides the stored value; dragging it previously
  appeared to do nothing.
- Correct the Appearance docs sentence for the topbar quick switch (it listed
  fonts; the quick switch now carries visual style, readability, and text size).

frontend/src/components/mobile/MobileFleet.tsx

@@ -198,7 +198,7 @@ function NodeDetail({
         <span aria-hidden className={`absolute left-0 top-1 bottom-[15px] w-[3px] ${tone === 'destructive' ? 'bg-destructive' : tone === 'warning' ? 'bg-warning' : 'bg-brand'}`} />
         <div className="mb-1"><Kicker>{`fleet › node › ${node.name}`}</Kicker></div>
         <div className="flex items-center gap-2.5">
-          <span className="min-w-0 truncate font-display italic text-[30px] leading-[34px] text-stat-value">{node.name}</span>
+          <span className="min-w-0 truncate font-heading text-[30px] leading-[34px] text-stat-value">{node.name}</span>
           <StatePill tone={tone} live={!online}>{node.cordoned ? 'cordoned' : online ? (isCritical(node) ? 'degraded' : 'online') : 'offline'}</StatePill>
         </div>
         <div className="mt-[7px] font-mono text-[12px] text-stat-subtitle">

frontend/src/components/mobile/MobileSettings.tsx

@@ -51,7 +51,7 @@ export function MobileSettings({ headerActions }: MobileSettingsProps) {
         <div className="relative border-b border-hairline px-4 pb-[15px] pt-1">
           <span aria-hidden className="absolute left-0 top-1 bottom-[15px] w-[3px] bg-brand" />
           <div className="mb-1"><Kicker>{`settings · ${group?.label ?? ''} · ${item.label}`}</Kicker></div>
-          <span className="font-display italic text-[30px] leading-[34px] text-stat-value">{item.label}</span>
+          <span className="font-heading text-[30px] leading-[34px] text-stat-value">{item.label}</span>
         </div>
         <div className="flex-1 min-h-0 overflow-y-auto [scrollbar-width:none] [&::-webkit-scrollbar]:hidden px-4 pb-8 pt-4 flex flex-col gap-6">
           <SettingsSectionContent sectionId={activeSection} onDirtyChange={NOOP} showDescription />

frontend/src/components/mobile/mobile-ui.tsx

@@ -194,7 +194,7 @@ export function Masthead({
           <div className="mb-1">{kickerSlot ?? <Kicker>{kicker}</Kicker>}</div>
           <div className="flex items-center gap-[11px]">
             <StateDot tone={stateTone} size={11} pulse={live} />
-            <span className={cn('font-display italic text-[38px] leading-[40px] text-stat-value', stateClassName)}>{state}</span>
+            <span className={cn('font-heading text-[38px] leading-[40px] text-stat-value', stateClassName)}>{state}</span>
           </div>
         </div>
         {right ? <div className="shrink-0 text-right">{right}</div> : null}