Add a column to display estimated expiration date

[zire]

This PR tries to display the estimated expiration date directly in the user's dashboard table as the fourth column to the right.

• The original code displays only 3 columns: Canister, Allocated, and Donated.
• The user has to click on the specific canister to open up a collapsed menu to see more details.
• Among those details, the estimated days remaining (before cycles run out) is probably the most vital statistic for the user.
• This PR would display this statistic (but in the transformed format of July 7, 2026) directly in the dashboard table, without the user having to click on the canister ID.
• The effect looks like this on a local replica:
  
• The original .gitignore leaves room for some environmental or artifact files. I did not stage those for my commit. This is supposed to be a simple UI change. The only affected files should be the index.html and index.js, I think.
  

[ninegua]

This name is a bit long, the table won't look as nice on mobile phone. How about "Est. TTL"? TTL = time to live

[zire]

Agree. Est. TTL sounds good. The original col header is indeed too long, even on the desktop.