Administration: Fix Tags screen UI inconsistencies on add/delete

[SainathPoojary]

This PR fixes:

• Display the "No tags found." message immediately when the last tag is deleted via JavaScript.
• Hide .tablenav sections when no tags are present.
• Show .tablenav sections when a new tag is added via JS if none previously existed.
• Ensure pagination count is updated dynamically when tags are added or removed.

Trac ticket: #63372

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[github-actions]

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[github-actions]

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Props sainathpoojary, joedolson.

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[joedolson]

I made some refinements to this, but I'm in doubt whether it will be ready in time for 6.9. One issue was that the previous force push had overwritten some previous accessibility improvements; you'll want to be a bit more careful when merging prior changes in.