update terminology from 'stream' to 'channel'

[Gopinath-Mahendiran]

What does this PR do, and why?

This PR introduces support for dynamically updating UI terminology based on the current feature_level.

•	Refactored label and message text sources to respond to the feature_level value.
•	Introduced centralized terminology management logic to allow conditional term switching.
•	Ensured backward compatibility with previous terminology when feature_level is below the threshold.
•	Updated affected UI components to pull display terms dynamically instead of using hardcoded strings.

Outstanding aspect(s)

still need to change the code base terminologies

External discussion & connections

• [ x] Discussed in #zulip-terminal in topic
• Fully fixes #
• [ x] Partially fixes issue Migrate from 'Stream' to 'Channel' naming #1510
• Builds upon previous unmerged work in PR #
• Is a follow-up to work in PR #
• Requires merge of PR #
• [x ] Merge will enable work on # code base terminology changes

How did you test this?

• [ x] Manually - Behavioral changes
• [ x] Manually - Visual changes
• [x ] Adapting existing automated tests
• Adding automated tests for new behavior (or missing tests)
• [ x] Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• [ x] It is a minimal coherent idea
• [ x] It has a commit summary following the documented style (title & body)
• [x ] It has a commit summary describing the motivation and reasoning for the change
• It individually passes linting and tests
• It contains test additions for any new behavior
• It flows clearly from a previous branch commit, and/or prepares for the next commit

Visual changes

[Gopinath-Mahendiran]

@neiljp