Update loading state for Content Summarization

[jeffpaul]

Description

Let's update the loading state of the Generate Summary button to Generating... and then after to Re-generate Summary to match most other AI plugin buttons loading states of "Generating...".

Step-by-step reproduction instructions

content-summarization-loading.mp4

Screenshots, screen recording, code snippet

No response

Environment info

No response

Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except the AI plugin.

• Yes

Please confirm which theme type you used for testing.

• Block
• Classic
• Hybrid (e.g. classic with theme.json)
• Not sure

[dkotter]

The latter part of this is how it already works (button updates to Re-generate Summary after summarization is finished) but opened #382 to update the loading text to say Generating...:

summarization.mov