[ticket 640] added nickname to note

[AleksPlekhov]

Description

Shows the user's nickname above the location note input field so they know which identity they're posting with before submitting.

Previously, when composing a location note, there was no indication of which name the note would be attributed to. Now a small @Nickname label appears above the text field, using the same monospace style as the note author headers in the list above — so the format feels consistent.

Closes: #640

Checklist
[x] I have read the contribution guidelines: https://github.com/permissionlesstech/bitchat-android?tab=readme-ov-file#contributing
[x] I have performed a self-review of my code
[x]I have mentioned the corresponding issue and the relevant keyword (e.g., "Closes: #xy") in the description (see https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue)
[] If it is a core feature, I have added automated tests

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