[ui] Make "Share on Nostr" a working button

The share menu's "Share on Nostr" entry was never clickable, no matter who was
logged in -- it rendered as inert, dimmed text.

VideoModal initialized `shareNostrAuthState` to { isLoggedIn: false, hasSigner:
false } and nothing ever wrote to it. createVideoShareMenuPanel gates the item
on `isLoggedIn && hasSigner`, so it was always disabled, and the click
delegation in bindShareMenuActions() bails on `button.disabled` -- so the whole
share-to-Nostr flow behind it was unreachable.

The renderer and the publish path were both already correct; only the gate was
missing:

- VideoModal.setShareNostrAuthState() records the state and closes an already
  open share menu when it changes, so a menu opened before login does not keep
  showing the stale disabled item. It is a no-op when nothing changed, to avoid
  slamming the menu shut on every sync.
- syncAuthUiState() computes the gate from isUserLoggedIn() plus a
  signing-capable active signer, mirroring ShareNostrController.handleShare():
  a signer without signEvent would only produce an error toast on click.
- syncAuthUiState() was only ever called at bootstrap, so logging in
  mid-session left the gate closed. It now self-installs a bitvid:auth-changed
  listener on first run (bound once, re-entrancy safe) rather than adding a
  call site in js/app.js, which is at its file-size cap.

Verified in a browser that the rendered item is disabled when logged out or
signer-less and enabled + click-dispatching with both; the new tests fail 5/9
with the gate push removed.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

js/ui/components/VideoModal.js

@@ -3453,6 +3453,32 @@ export class VideoModal {
   // Delegated click handler for the share popover's menu-action buttons
   // (Copy URL / Copy Magnet / Copy CDN / Share on Nostr). The forced-source
   // test links self-wire their own copy, so they are skipped here.
+  /**
+   * Update the login/signer gate for the share menu's "Share on Nostr" item.
+   *
+   * This state was initialized to `false` and never written, so
+   * `createVideoShareMenuPanel` always rendered the item disabled — the entry
+   * looked like inert text no matter who was logged in, and the click
+   * delegation in bindShareMenuActions() bails on `button.disabled`. The app
+   * pushes real state here from syncAuthUiState().
+   */
+  setShareNostrAuthState({ isLoggedIn = false, hasSigner = false } = {}) {
+    const next = { isLoggedIn: !!isLoggedIn, hasSigner: !!hasSigner };
+    const changed =
+      next.isLoggedIn !== this.shareNostrAuthState.isLoggedIn ||
+      next.hasSigner !== this.shareNostrAuthState.hasSigner;
+    this.shareNostrAuthState = next;
+
+    // The popover renders its panel on open, so a menu that is already open
+    // would keep showing the stale (disabled) item. Close it so the next open
+    // renders against current state.
+    if (changed) {
+      this.modalSharePopover?.close?.();
+    }
+
+    return changed;
+  }
+
   bindShareMenuActions(panel) {
     if (!panel || typeof panel.addEventListener !== "function") {
       return;