refactor(design): migrate hardcoded colors to kami tokens

Purge remaining hardcoded color values from the component layer and main
process so every surface flows through the kami token system.

src/main/desktop-shell-window.ts
  - BrowserWindow.backgroundColor no longer paints pure white before the
    renderer mounts. It now picks kami parchment (#F5F4ED) or deep-dark
    (#141413) based on nativeTheme.shouldUseDarkColors so the startup
    flash respects the user's OS theme preference.

src/shared/constants.ts
  - DEPARTMENT_COLORS collapses 12 unrelated hex brand colors (violet,
    red, green, cyan, pink, ...) into six kami tokens: var(--primary),
    var(--kami-ink), var(--kami-dark-warm), var(--kami-charcoal),
    var(--kami-olive), var(--kami-stone). Departments are distinguished
    by warm-neutral ramp + ink-blue, never by second chromatic hue.

src/renderer/components/communication/{MessageItem,SearchResults}.tsx
  - Replace the string-concat tint pattern `${color}18` with
    color-mix(in oklab, ${color}, transparent 91%). The old pattern
    produced invalid CSS (e.g. `var(--primary)18`) whenever `color`
    was a CSS custom property, silently falling back to no background.
    color-mix works for both hex and var() inputs, eliminating the
    latent bug at the source.

src/renderer/components/ui/ActionButton.tsx
  - Replace hand-rolled rgba(74,93,122,0.35) and rgba(220,53,69,0.3)
    drop shadows with kami ring shadows routed through color-mix over
    var(--primary) and var(--destructive).

src/renderer/views/CommandPaletteView.tsx
  - Replace rgba(15,23,42,0.08) cold-slate shadow with a kami whisper
    shadow driven by color-mix over var(--foreground).

src/renderer/components/skills/SkillAgentToggles.tsx
src/renderer/components/execution/ModelManagementShared.tsx
  - Toggle thumbs switch from bg-white to bg-background so they
    inherit parchment in light mode and deep-dark in dark mode,
    removing the last pure-white surfaces from the renderer.

Third-party brand hex values in src/renderer/components/social/
social-config.ts are retained: they represent external brand identity
(Twitter blue, YouTube red, ...), not kami chrome, and collapsing them
to one hue would destroy recognition.

Typecheck clean. No business-logic signatures changed.

src/renderer/components/execution/ModelManagementShared.tsx

@@ -77,7 +77,7 @@ export function ToggleSwitch({ checked, onChange, label }: { checked: boolean; o
       aria-label={label}
     >
       <span
-        className={`inline-block h-3.5 w-3.5 rounded-full bg-white shadow-sm transition-transform ${checked ? "translate-x-[18px]" : "translate-x-[3px]"}`}
+        className={`inline-block h-3.5 w-3.5 rounded-full bg-background shadow-sm transition-transform ${checked ? "translate-x-[18px]" : "translate-x-[3px]"}`}
       />
     </button>
   );