stream_settings: Fix jitter behavior of plus/checkmark.

[sujalshah-bit]

Previously, we show loading spinner in each streams rows. Now we show directly plus or checkmark icon instead of loading spinner.

CZO

Screenshots and screen captures:
Before:

After:

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[alya]

@sujalshah-bit please ping your buddy for review.

[adnan-td]

LGTM but I have noticed a related bug:

When clicking on the subscribe button, the add button becomes enabled (from disabled). This still feels like jitter as transition from disabled to enabled is visible.