Screenshots: Downgrading a plan (4 placeholders)

[superdav42]

For #64.

Problem

docs/user-guide/client-management/downgrading-a-plan.md references 4 distinct concepts across two recycled images.

docs/user-guide/client-management/downgrading-a-plan.md
  L16 ![Account page with Change button under membership](/img/admin/memberships-list.png)
  L20 ![Checkout page showing plan options for downgrade](/img/admin/memberships-list.png)
  L26 ![Account page showing pending membership change message](/img/admin/memberships-list.png)
  L38 ![Post limit exceeded options in Ultimate Multisite settings](/img/config/settings-sites.png)

Captures required

Setup: a customer subscribed to a higher-tier plan with a lower-tier plan available in the same plan group. From the customer subsite, initiate a downgrade; do not finalise it so the pending change state is visible.

#	Doc line	Capture target	New filename
1	L16	Customer account page Your Membership card with Change button (share filename with upgrading-a-plan.md issue)	static/img/account-page/membership-change-button.png
2	L20	Downgrade plan-options page on the customer side	static/img/account-page/downgrade-picker.png
3	L26	Account page showing the pending membership change banner	static/img/account-page/pending-change.png
4	L38	Network Admin → Settings → Sites: Post-limit-exceeded behaviour options	static/img/config/settings-sites-post-limit.png

Capture at 1280×800, crop to the relevant card. < 200 KB.

Doc edit

Update every line in docs/user-guide/client-management/downgrading-a-plan.md to its new path and refine alt text.

Done when

• All 4 new PNGs added.
• docs/user-guide/client-management/downgrading-a-plan.md updated.
• Customer-side vs admin-side captures distinguishable.
• PR opened with Resolves for this issue and Ref #64.