feat: keep the Add account action visible while scrolling

[vytisbulkevicius]

Summary

When many social accounts are connected, the "Add account" action was only available at the very bottom of the Accounts list, making it hard to find as the list grows. This makes the existing "Add account" element sticky so it stays visible while scrolling a long list. It only activates with 2+ accounts; with a short list there is no visual change. No duplicated component or auth logic.

Will affect the visual aspect of the product

YES — the "Add account" bar sticks to the bottom of the accounts list while scrolling.

Test instructions

• Connect 2+ accounts so the Accounts list is long enough to scroll.
• Scroll the Accounts tab and confirm the "Add account" bar stays pinned at the bottom of the view.
• With 0–1 accounts (short list), confirm nothing changed.

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR

Note: this is a presentational, CSS-only change (a position: sticky toggle on the "Add account" action). There is no server-side logic to unit test, the project has no JS/Vue unit-test harness, and an e2e assertion for scroll-stickiness would be flaky, so no automated test is included. Verified manually in the dashboard.

Closes #1070

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[vytisbulkevicius]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Merge conflicts resolved. The only conflict was in the built assets/js/build/dashboard.js — I merged origin/development into the branch, rebuilt from the merged sources (so the output includes both the sticky Add Account changes and the Edit Post button from development), and committed the result.