feat(widget): plug-and-play governance UI as custom elements

Follows bitlogin's layering: the engine stays headless and the widget is a
consumer of the published API, never a dependency of it. Building it was also a
test of that API — an element needing something unexposed would have been a
finding, the way the commerce example was.

Viewer elements: governance-veil (blur on restrict, disclosure on hide, warning
on warn), governance-report, governance-status. Moderator elements:
governance-capabilities, governance-action, governance-admin-panel.

Deliberate behaviors:
- hidden content leaves the accessibility tree rather than being visually
  covered, so a screen reader does not read out what the viewer must not see
- a hidden decision renders a disclosure, not nothing: content that silently
  vanishes is indistinguishable from a bug and leaves no route to appeal
- refused moderator actions stay visible and name the missing capability. A
  hidden button teaches nothing about the authority model, and the command
  layer enforces the same check independently — the UI is an explanation,
  never the gate
- no element computes policy; wording maps from stable reason identifiers and
  is overridable per application

Found while building it: GovernanceAdminStore emitted only when reduced denial
state changed, so revoking a role that denied nobody announced nothing, leaving
capability-gated UI and the decision cache stale.

The root tsconfig deliberately omits the DOM lib — that absence is what keeps
core, nostr, and runtime from reaching for browser globals — so the widget
carries its own DOM-aware config rather than relaxing the guarantee globally.

687 tests; lint, typecheck, and the declaration build pass from a cold checkout.

package.json

@@ -9,10 +9,10 @@
     "test:unit": "vitest run --passWithNoTests",
     "test:conformance": "vitest run tests/conformance --passWithNoTests",
     "typecheck": "tsc -p tsconfig.json --noEmit",
-    "build:types": "tsc -p packages/core/tsconfig.types.json && tsc -p packages/nostr/tsconfig.types.json && tsc -p packages/runtime/tsconfig.types.json && tsc -p packages/bitvid-compat/tsconfig.types.json && tsc -p packages/testing/tsconfig.types.json",
+    "build:types": "tsc -p packages/core/tsconfig.types.json && tsc -p packages/nostr/tsconfig.types.json && tsc -p packages/runtime/tsconfig.types.json && tsc -p packages/bitvid-compat/tsconfig.types.json && tsc -p packages/widget/tsconfig.types.json && tsc -p packages/testing/tsconfig.types.json",
     "lint": "npm run check:references && npm run check:types",
     "check:references": "node scripts/verify-no-reference-imports.mjs",
-    "check:types": "npm run typecheck"
+    "check:types": "tsc -p tsconfig.json --noEmit && tsc -p packages/widget/tsconfig.json --noEmit"
   },
   "engines": {
     "node": ">=22"
@@ -23,6 +23,7 @@
   ],
   "devDependencies": {
     "@types/node": "^22.10.0",
+    "happy-dom": "^20.11.2",
     "typescript": "^5.4.5",
     "vitest": "^1.6.0"
   }