fix(dm): can't open conversations + "helpers unavailable" on a working signer

After switching to a responsive signer (nos2x), two DM-specific bugs surfaced.

1. Clicking a conversation in the list threw
   "this.controller.handleDmConversationSelect is not a function"
   (ProfileDirectMessageRenderer onSelectConversation). The method exists on
   ProfileDirectMessageActions and the controller delegates ~15 sibling Actions
   methods — but the handleDmConversationSelect delegation was missing. Add it.

2. Loading a conversation's messages threw "DM decryption helpers are
   unavailable" even though the signer decrypts blocks/subscriptions/hashtags
   fine. buildDmDecryptContext gated decryptor registration on (a) the dynamic
   `capabilities` getter (which probes window.nostr and false-negatives on lazy
   injection / nos2x's module shape) and (b) the permission-probe result. The
   list services already learned to register by METHOD EXISTENCE and validate at
   call time; align the DM path to do the same so a present, working signer
   never yields zero decryptors. The permission probe still runs.

Tests: tests/dm-decrypt-context.test.mjs (cheat-resistant — a signer whose
capabilities getter reports false and whose permission probe returns ok:false
must still register nip04+nip44 decryptors). Full unit + dm:unit + dm:integration
suites green. KNOWN_BUGS #0.

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

js/ui/profileModal/ProfileDirectMessageController.js

@@ -599,6 +599,10 @@ export class ProfileDirectMessageController {
     return this.actions.pauseProfileMessages(...args);
   }
 
+  handleDmConversationSelect(...args) {
+    return this.actions.handleDmConversationSelect(...args);
+  }
+
   handleSendDmRequest(...args) {
     return this.actions.handleSendDmRequest(...args);
   }