fix(media): wait for the session instead of failing media requests at startup

[eleboucher]

Description

Media failed to load until the view was dismissed and reopened. Requests arriving before the frontend handed the session to Rust were answered 503, and <img> never retries, so media stayed broken until the component remounted. The protocol now waits briefly for the session, and still fails fast once a session has existed so requests in flight during a logout do not hang.

Also raises the media download lane from 2 to 6 concurrent, so a sticker grid does not load two at a time.

Fixes #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).