fix(desktop): auto-link device on sign-in + publish key on accept

Two bugs reported by the user:

1. After accepting a contact request on the desktop, the requester's
   "Send message" call errored with "no encryption key published" for
   the newly-accepted contact. Root cause: desktop never ran the
   device-registry bootstrap (mobile does it from _layout.tsx on
   sign-in) — so the desktop's X25519 pub was never published via
   LINK_DEVICE, and resolveRecipientKeys returned an empty list.

2. On the accepting device, the new chat didn't appear in Messages
   after tapping Accept — accept wrote the contact to store + disk
   but didn't switch sections, so the user was stuck in Contacts
   watching nothing happen.

Fixes:

  * hooks/useDeviceBootstrap — direct port of mobile's _layout.tsx
    bootstrap effect. On every sign-in:
      - fetchDevices(master) → if our X25519 is listed, mark local
        registered flag.
      - not listed + was registered before → REVOKED → wipe state +
        bounce to Welcome.
      - not listed + never registered → submit LINK_DEVICE. Tx may
        bounce if balance is zero; next launch retries.
    Mounted from App.tsx so it runs once per authenticated session.

  * RequestsList.accept — after submitting ACCEPT_CONTACT, check if
    OUR X25519 is in the on-chain registry. If not, submit LINK_DEVICE
    immediately (balance is now covered by the contact fee the peer
    paid us). This closes the window where the peer couldn't encrypt
    to us because our key wasn't published yet.
    Also: after a successful accept, setSection('messages') +
    setActiveChat(requester_pub), matching mobile's
    router.replace('/chats/<pub>') flow.

  * Conversation.send — nicer error copy when
    resolveRecipientKeys returns []. Was: "recipient has no
    encryption key published". Now: actionable text asking the peer
    to re-open their app so the LINK_DEVICE tx commits.

desktop/src/sections/messages/Conversation.tsx

@@ -65,7 +65,15 @@ export function Conversation({ address }: { address: string }): React.ReactEleme
       if (!isSelf) {
         const pubs = await resolveRecipientKeys(address);
         if (pubs.length === 0) {
-          throw new Error('recipient has no encryption key published');
+          // Most common cause: the peer's device hasn't published a
+          // LINK_DEVICE yet (they accepted just now and haven't had the
+          // fee debited, or they haven't re-opened the app). Clearer
+          // copy than "recipient has no encryption key".
+          throw new Error(
+            'Recipient has no device key published on-chain yet. ' +
+            'Ask them to re-open their app so the LINK_DEVICE tx commits, ' +
+            'then try again.',
+          );
         }
         await Promise.all(pubs.map(async (rpub) => {
           const { nonce, ciphertext } = encryptMessage(