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/plugins/registry.ts

@@ -28,6 +28,7 @@ import { Select, SelectOption } from "@/components/ui/select";
 import { Separator } from "@/components/ui/separator";
 import { Tabs, TabsList, TabsTrigger } from "@/components/ui/tabs";
 import { useI18n } from "@/i18n";
+import { registerSlot, PluginSlot } from "./slots";
 
 // ---------------------------------------------------------------------------
 // Plugin registry — plugins call register() to add their component.
@@ -75,6 +76,7 @@ declare global {
     __HERMES_PLUGIN_SDK__: unknown;
     __HERMES_PLUGINS__: {
       register: typeof registerPlugin;
+      registerSlot: typeof registerSlot;
     };
   }
 }
@@ -82,6 +84,7 @@ declare global {
 export function exposePluginSDK() {
   window.__HERMES_PLUGINS__ = {
     register: registerPlugin,
+    registerSlot,
   };
 
   window.__HERMES_PLUGIN_SDK__ = {
@@ -118,6 +121,7 @@ export function exposePluginSDK() {
       Tabs,
       TabsList,
       TabsTrigger,
+      PluginSlot,
     },
 
     // Utilities