feat: add campaign settings modal save action

[saurabhhhcodes]

Closes #216\n\nAdds an explicit save action to the campaign settings modal, keeps the existing settings fields wired into campaign persistence, and lets the gear icon open a usable settings workflow from the campaign builder top bar.\n\nValidation:\n- git diff --check\n- headless Chrome render check against http://127.0.0.1:4174/campaign-builder.html\n

[Kuldeeep18]

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