nip71: account-level "auto-share new public videos" toggle

Adds an opt-in account preference (off by default, per pubkey) that
auto-mirrors newly published eligible public videos to NIP-71
(kinds 34235/34236) so they surface on other Nostr video clients —
no per-video opt-in needed.

- nip71MirrorFlags: isAutoShareEnabled/setAutoShareEnabled (persisted
  per pubkey) + pure resolvePublishSync({featureOn,autoShare,eligible}).
- nip71MirrorSync: syncNip71MirrorAfterPublish + a videos:published
  subscription in initNip71MirrorSync; converts the canonical 30078
  event, checks eligibility (public + hosted URL + ALLOW_NSFW gate),
  publishes the mirror, and records the per-video flag so later
  edits/deletes stay in lockstep. All best-effort.
- My Videos pane: an "Auto-share new public videos to other Nostr apps"
  checkbox wired to setAutoShareEnabled, gated by FEATURE_NIP71_MIRROR.

Tests (mutation-verified): auto-share persistence, resolvePublishSync
(eligible+on=>publish; off/ineligible=>none), and videos:published
subscription wiring.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

components/profile-modal.html

@@ -1028,6 +1028,10 @@
                 role="status"
                 aria-live="polite"
               ></p>
+              <label id="profileMyVideosAutoShareRow" class="hidden flex items-start gap-2 text-sm text-muted-strong">
+                <input id="profileMyVideosAutoShare" type="checkbox" class="checkbox mt-0.5" />
+                <span>Auto-share new public videos to other Nostr apps (NIP-71)</span>
+              </label>
               <p
                 id="profileMyVideosLoading"
                 class="text-sm text-muted hidden"