Fix folder sync for contacts without a public phone number

[huzvanec]

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Description

When adding a user without a public phone number to a folder on mobile, the contact did not sync to desktop. The recipientToConversationId function relied on Recipient.contact.serviceId, which is often (always during my testing) undefined.
This led to all contacts falling back to being identified by their phone numbers (e164) and contacts without a public phone number caused an error and didn't get added to the folder.

This PR fixes this by identifying contacts primarily by their serviceIdBinary which is always present.

Tested on Android 16 -> Debian GNU/Linux 13 (trixie) x86_64 (against prod servers).

Example error raised for contacts without public phone numbers:

ERROR 2026-02-28T21:40:29.292Z [storage] merge(121:r5z): error with item type=8 details=Error: mergeChatFolderRecord(121:r5z, idString): Missing conversation for recipient
    at strictAssert ([REDACTED]/resources/app.asar/preload.bundle.js:23:570)
    at recipientToConversationId ([REDACTED]/resources/app.asar/preload.bundle.js:169:176953)
    at [REDACTED]/resources/app.asar/preload.bundle.js:169:177040
    at Array.map (<anonymous>)
    at recipientsToConversationIds ([REDACTED]/resources/app.asar/preload.bundle.js:169:177033)
    at mergeChatFolderRecord ([REDACTED]/resources/app.asar/preload.bundle.js:169:177561)
    at mergeRecord ([REDACTED]/resources/app.asar/preload.bundle.js:169:199496)
    at concurrency ([REDACTED]/resources/app.asar/preload.bundle.js:169:207760)
    at [REDACTED]/resources/app.asar/preload.bundle.js:54:845
    at process.processTicksAndRejections (node:internal/process/task_queues:103:5)

[jamiebuilds-signal]

Cherry picking this into our next release, thank you