widget: ship generated type declarations + typed element/events

Integrators had to read the demo to learn the API: the package built via
vite with no .d.ts and no types field, so @bitlogin/widget and
@bitlogin/widget/session resolved to `any`.

Emit declarations from source (tsconfig.types.json -> dist-types,
build:types wired into build) and point the package types/exports
conditions at them, for both the root and the /session entry. Generated,
so they can't drift from the implementation.

Add the type surface integrators actually want:
- HTMLElementTagNameMap["bitlogin-auth"], so querySelector/createElement
  return the typed element.
- Typed addEventListener/removeEventListener overloads (declaration-merged
  interface) for the bitlogin-* events.
- Exported event-detail types and BitLoginEventMap (src/events.ts), matching
  element.ts's dispatchEvent shapes.

Verified with a consumer conformance typecheck (typed detail with no cast,
typed element methods, readCachedSession return type). README gains a
"TypeScript" section.

Addresses the ".d.ts / documented API surface" item of the Tessera
integration feedback (docs/TODO.md).

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01BXQCvJPA6sVSQ5j8uBd9bB

packages/widget/package.json

@@ -6,12 +6,20 @@
   "type": "module",
   "description": "BitLogin embeddable login widget: a <bitlogin-auth> custom element plus a window.nostr NIP-07-compatible provider, backed by a dedicated crypto Web Worker.",
   "main": "./dist/bitlogin.js",
+  "types": "./dist-types/index.d.ts",
   "exports": {
-    ".": "./dist/bitlogin.js",
-    "./session": "./dist/bitlogin-session.js"
+    ".": {
+      "types": "./dist-types/index.d.ts",
+      "default": "./dist/bitlogin.js"
+    },
+    "./session": {
+      "types": "./dist-types/session.d.ts",
+      "default": "./dist/bitlogin-session.js"
+    }
   },
   "scripts": {
-    "build": "vite build",
+    "build": "vite build && npm run build:types",
+    "build:types": "tsc -p tsconfig.types.json",
     "dev": "vite build --watch",
     "typecheck": "tsc --noEmit"
   },