Add BitLogin as the primary sign-in method (gated, unstable)

The login modal now shows <bitlogin-auth> -- a portable Nostr identity
behind a login name and password -- as the default sign-in path,
styled with bitvid's own brand tokens (accent-strong, surfaces, radii).
The existing NIP-07 extension, nsec, and NIP-46 remote-signer options
still work exactly as before, just collapsed under a new "Other login
options" fold instead of a flat "Available methods" grid; nip07 no
longer carries the stale "Recommended" eyebrow now that BitLogin has
that role.

BitLogin isn't a single button-click-then-await-a-promise flow like
the other providers -- it's a whole multi-screen sign-in/create/recover
UI -- so it gets a permanent, statically-declared widget in
components/login-modal.html (js/ui/bitloginModalIntegration.js relocates
it, never destroys and recreates it, since removal tears down its crypto
Web Worker) instead of a grid button. It still goes through the exact
same session-establishment path as every other signer: a new "bitlogin"
provider (js/services/authProviders/bitlogin.js) lets AuthService
resolve and activate it via the normal requestLogin -> handleAuthLogin
path, while providersForModal excludes it from the grid so it isn't
rendered twice.

Two things this needed beyond the wiring itself:
- js/app.js is one line from its lint/file-size ceiling, so the only
  change there is renaming an import binding (providers -> providersForModal),
  a net-zero line change. The actual widget/event wiring lives in a new,
  unconstrained module instead.
- The login modal's HTML (including the permanent widget element) loads
  asynchronously after bootstrap runs, so wireBitloginLogin() waits via
  MutationObserver rather than assuming a specific call order.

Vendored via scripts/build-bitlogin-widget.mjs (mirrors the existing
Bitcoin Connect/Blossom vendor steps): BitLogin isn't published to npm,
so the widget build is copied verbatim from the sibling ../bitlogin
checkout into committed vendor/bitlogin/, then lazy-imported (like
Bitcoin Connect) so its ~350KB never weighs on visitors who never open
the login modal.

Gated behind FEATURE_BITLOGIN (config/instance-config.js), default on
for this unstable-branch experiment per AGENTS.md §1 -- reset to false
before this reaches main until verified end-to-end against production
relays.

Verified: npm run lint (clean), the full auth/login test suite (43/43,
including new coverage for the provider + adapter), and a manual
Chrome pass against the built dist/ -- widget renders themed correctly,
"Other login options" still shows the original four methods unchanged.

scripts/build-bitlogin-widget.mjs

@@ -0,0 +1,68 @@
+// Vendor the sibling @bitlogin/widget build into a pinned, same-origin bundle.
+//
+// bitvid's CSP is script-src 'self' (plus a couple of pinned CDN fallbacks for
+// nostr-tools) -- the same reasoning as the existing bitcoin-connect/blossom-sdk
+// vendor steps. Unlike those, BitLogin ships its own Vite-built ESM output
+// directly (bitlogin.js + cryptoWorker.js + a shared chunk they both import),
+// so this script copies that output verbatim instead of re-bundling it with
+// esbuild -- the worker's relative self-URL resolution depends on the three
+// files staying exactly as built and sitting side by side (see BitRoad's and
+// BitUnlock's identical vendor steps for this same reasoning).
+//
+//   node scripts/build-bitlogin-widget.mjs
+//   BITLOGIN_ENTRY=/path/to/bitlogin node scripts/build-bitlogin-widget.mjs
+
+import { createHash } from "node:crypto";
+import { copyFile, mkdir, readdir, readFile, rm, writeFile } from "node:fs/promises";
+import { existsSync } from "node:fs";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import { execSync } from "node:child_process";
+
+const repoRoot = path.resolve(path.dirname(fileURLToPath(import.meta.url)), "..");
+const bitloginRoot = process.env.BITLOGIN_ENTRY
+  ? path.resolve(process.env.BITLOGIN_ENTRY)
+  : path.resolve(repoRoot, "../bitlogin");
+const widgetDir = path.join(bitloginRoot, "packages/widget");
+const widgetDist = path.join(widgetDir, "dist");
+const vendorDir = path.join(repoRoot, "vendor/bitlogin");
+const integrityFile = path.join(repoRoot, "vendor/bitlogin.integrity.json");
+
+const packageJson = JSON.parse(await readFile(path.join(widgetDir, "package.json"), "utf8"));
+if (packageJson.name !== "@bitlogin/widget") {
+  throw new Error("BITLOGIN_ENTRY must point to a BitLogin checkout (packages/widget/package.json not found there).");
+}
+
+if (!existsSync(widgetDist)) {
+  console.log("[build-bitlogin-widget] dist missing, building @bitlogin/core then @bitlogin/widget first...");
+  execSync("npm run build -w @bitlogin/core", { cwd: bitloginRoot, stdio: "inherit" });
+  execSync("npm run build -w @bitlogin/widget", { cwd: bitloginRoot, stdio: "inherit" });
+}
+
+await rm(vendorDir, { recursive: true, force: true });
+await mkdir(vendorDir, { recursive: true });
+
+const distEntries = (await readdir(widgetDist)).filter((name) => name.endsWith(".js"));
+if (!distEntries.includes("bitlogin.js") || !distEntries.includes("cryptoWorker.js")) {
+  throw new Error("Expected bitlogin.js and cryptoWorker.js in @bitlogin/widget dist -- did the build layout change?");
+}
+
+const files = {};
+for (const name of distEntries) {
+  await copyFile(path.join(widgetDist, name), path.join(vendorDir, name));
+  const bytes = await readFile(path.join(widgetDist, name));
+  files[name] = { sha256: createHash("sha256").update(bytes).digest("hex"), bytes: bytes.byteLength };
+}
+
+await writeFile(
+  integrityFile,
+  `${JSON.stringify({ package: packageJson.name, version: packageJson.version, files }, null, 2)}\n`,
+);
+
+try {
+  await copyFile(path.join(bitloginRoot, "LICENSE"), path.join(repoRoot, "vendor/bitlogin.LICENSE"));
+} catch (error) {
+  console.warn("[build-bitlogin-widget] could not copy LICENSE:", error.message);
+}
+
+console.log("[build-bitlogin-widget] wrote", path.relative(repoRoot, vendorDir), `(${distEntries.length} files)`);