Improve demo chat layout

[gVguy]

Some styles look off in the demo chat of Alice and Bob.

This PR includes following fixes:

• Alice's messages in Bob's screen are too wide (don't depend on message content)
• Input should be 100% width
• Sides where sent messages appear should be inverted — usually "my" messages are on the right.

Before:

After:

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[palkan]

Thanks!