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.

config/instance-config.js

@@ -276,6 +276,19 @@ export const FEATURE_AUDIO_INGEST = false;
  */
 export const FEATURE_BITCOIN_CONNECT = true;
 
+/**
+ * Whether the login modal offers BitLogin (a portable Nostr identity behind a
+ * login name/password, https://github.com/PR0M3TH3AN/bitlogin) as the primary
+ * sign-in method, with the NIP-07 extension, nsec, and NIP-46 options
+ * collapsed under "Other login options" (js/ui/bitloginModalIntegration.js).
+ * The vendored widget bundle (vendor/bitlogin/, scripts/build-bitlogin-widget.mjs)
+ * only lazy-loads once the login modal's HTML is present in the DOM.
+ *
+ * Unstable-branch experiment (AGENTS.md §1) -- reset to `false` before this
+ * reaches main until verified end-to-end against production relays.
+ */
+export const FEATURE_BITLOGIN = true;
+
 /**
  * Whether this deployment offers Blossom (nostr-native blob storage) as a
  * first-class upload provider alongside Cloudflare R2 / generic S3 / Backblaze B2