chore(sales-dashboard): add multiselect to plan filter

[matthewelwell]

Changes

Adds multiselect to the plan filter in the sales dashboard.

The reasoning behind doing this is to remove the free plan by default to reduce the impact of loading the sales dashboard.

How did you test this code?

Ran the sales dashboard locally and validated the behaviour.

[vercel]

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Project	Deployment	Preview	Comments	Updated (UTC)
flagsmith-frontend-preview	Ready	Preview	Comment	Nov 5, 2025 10:01am
flagsmith-frontend-staging	Ready	Preview	Comment	Nov 5, 2025 10:01am

1 Skipped Deployment

Project	Deployment	Preview	Comments	Updated (UTC)
docs	Ignored	Preview		Nov 5, 2025 10:01am

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-6243	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-6243	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-6243	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6243	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6243	Finished ✅	Results ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.01%. Comparing base (3f8b260) to head (8d0e77a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6243   +/-   ##
=======================================
  Coverage   98.01%   98.01%           
=======================================
  Files        1278     1278           
  Lines       45178    45183    +5     
=======================================
+ Hits        44280    44285    +5     
  Misses        898      898           

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