Use max_topic_length instead of hardcoded limit of 60

[AhmedTareek]

Fixes: #307

Changes

• Added maxTopicLength field to the initial snapshot to retrieve the value from the server.
• Stored maxTopicLength in PerAccountStore from the initial snapshot.
• Updated ComposeTopicController in compose_box to use the value from PerAccountStore.
• Removed the hardcoded value which was in route/messages.dart
• Modified app_en.arb to dynamically display the maxTopicLength value in the topic length error message, replacing the hardcoded "60"
• Changed the _send method of compose_box to use the modified error message that contains the dynamic maxTopicLength

Testing

• Updated tests to use the new value from PerAccountStore instead of the hardcoded value.
• Confirmed the new behavior by running existing tests with multiple maxTopicLength variants.

[chrisbobbe]

Chat thread: #mobile-team > flutter app and max_topic_length @ 💬

[AhmedTareek]

Tests are added and I think it's ready for review now.

[chrisbobbe]

Thanks! Small comments below, and please organize your commits so they're clear and coherent according to the project style: https://github.com/zulip/zulip-mobile/blob/master/docs/style.md#git-commits-commit-messages