feat(dashboard): reskin extension points for themes and plugins (#14776)

Themes and plugins can now pull off arbitrary dashboard reskins (cockpit
HUD, retro terminal, etc.) without touching core code.

Themes gain four new fields:
- layoutVariant: standard | cockpit | tiled — shell layout selector
- assets: {bg, hero, logo, crest, sidebar, header, custom: {...}} —
  artwork URLs exposed as --theme-asset-* CSS vars
- customCSS: raw CSS injected as a scoped <style> tag on theme apply
  (32 KiB cap, cleaned up on theme switch)
- componentStyles: per-component CSS-var overrides (clipPath,
  borderImage, background, boxShadow, ...) for card/header/sidebar/
  backdrop/tab/progress/badge/footer/page

Plugin manifests gain three new fields:
- tab.override: replaces a built-in route instead of adding a tab
- tab.hidden: register component + slots without adding a nav entry
- slots: declares shell slots the plugin populates

10 named shell slots: backdrop, header-left/right/banner, sidebar,
pre-main, post-main, footer-left/right, overlay. Plugins register via
window.__HERMES_PLUGINS__.registerSlot(name, slot, Component). A
<PluginSlot> React helper is exported on the plugin SDK.

Ships a full demo at plugins/strike-freedom-cockpit/ — theme YAML +
slot-only plugin that reproduces a Gundam cockpit dashboard: MS-STATUS
sidebar with live telemetry, COMPASS crest in header, notched card
corners via componentStyles, scanline overlay via customCSS, gold/cyan
palette, Orbitron typography.

Validation:
- 15 new tests in test_web_server.py covering every extended field
- tests/hermes_cli/: 2615 passed (3 pre-existing unrelated failures)
- tsc -b --noEmit: clean
- vite build: 418 kB bundle, ~2 kB delta for slots/theme extensions

Co-authored-by: Teknium <p@nousresearch.com>

web/src/components/ui/card.tsx

@@ -1,12 +1,35 @@
 import { cn } from "@/lib/utils";
 
-export function Card({ className, ...props }: React.HTMLAttributes<HTMLDivElement>) {
+/**
+ * Themed card primitive. Themes can restyle every card without touching
+ * call sites by setting CSS vars under the `card` component-style bucket:
+ *
+ *   componentStyles:
+ *     card:
+ *       clipPath: "polygon(10px 0, 100% 0, 100% calc(100% - 10px), calc(100% - 10px) 100%, 0 100%, 0 10px)"
+ *       border: "1px solid var(--color-ring)"
+ *       background: "linear-gradient(180deg, var(--color-card) 0%, transparent 100%)"
+ *       boxShadow: "0 0 0 1px var(--color-ring) inset, 0 0 24px -8px var(--warm-glow)"
+ *
+ * All properties are optional — vars that aren't set compute to their
+ * CSS initial value, so the default shadcn-y card keeps looking normal
+ * for themes that don't override anything.
+ */
+const CARD_STYLE: React.CSSProperties = {
+  clipPath: "var(--component-card-clip-path)",
+  borderImage: "var(--component-card-border-image)",
+  background: "var(--component-card-background)",
+  boxShadow: "var(--component-card-box-shadow)",
+};
+
+export function Card({ className, style, ...props }: React.HTMLAttributes<HTMLDivElement>) {
   return (
     <div
       className={cn(
         "border border-border bg-card/80 text-card-foreground w-full",
         className,
       )}
+      style={{ ...CARD_STYLE, ...style }}
       {...props}
     />
   );