Implement Follow and Watch buttons for Subject and Article Page respectively

[eighttrigrams]

Problem

Users currently have no way to subscribe to updates for either a whole subject or a single article. This makes it difficult for them to stay informed about changes to the content they care about.

There are also two distinct notification needs that are not currently represented in the UI:

1. A user may want to follow an entire subject and receive updates related to that subject broadly.
2. A user may want to watch only a specific article and receive updates only for that article.

Without separating these two actions, users cannot control the scope of what they are subscribing to.

Solution

Add separate follow/watch subscription controls for subjects and articles:

• In the subject view, add a button that lets the user follow the subject.
• If the user is already following the subject, the button should display Following.
• In the article view, under the subject, add a toggle that lets the user watch that specific article.
• If the user is already watching the article, the toggle should display Unwatch.
• These two features should remain distinct:
• Following a subject subscribes the user to updates about the subject as a whole.
• Watching an article subscribes the user only to updates about that specific article.

This gives users clearer control over the level at which they want to receive updates.

[eighttrigrams]

relates to #135 and Pam's designs from this week