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/fleet/FleetMasthead.tsx

@@ -110,7 +110,7 @@ export function FleetMasthead({
             className={`h-2.5 w-2.5 rounded-full ${config.dotClass} ${level === 'healthy' ? '' : 'animate-[pulse_2.4s_ease-in-out_infinite]'}`}
           />
           <div className="flex flex-col gap-1">
-            <span className={`font-display italic text-3xl leading-none tracking-tight ${config.textClass}`}>
+            <span className={`font-heading text-3xl leading-none tracking-tight ${config.textClass}`}>
               {config.label}
             </span>
             <span className="font-mono text-[10px] leading-3 uppercase tracking-[0.18em] text-stat-subtitle">

frontend/src/components/fleet/RoutingTab.tsx

@@ -267,7 +267,7 @@ function RoutingMasthead({ meshedNodes, reachableNodes, totalAliases, onShowActi
     return (
         <div className="flex items-center justify-between rounded-lg border border-card-border bg-card p-4 shadow-card-bevel">
             <div className="flex items-center gap-4">
-                <div className="font-display italic text-2xl">{stateWord}</div>
+                <div className="font-heading text-2xl">{stateWord}</div>
                 <div className="grid grid-cols-3 gap-4 text-xs">
                     <div>
                         <div className="text-[10px] leading-3 tracking-[0.18em] uppercase text-stat-subtitle font-mono">meshed</div>