feat: add fab varients to conferencing and calendar page in mobile view

[KartikLabhshetwar]

What does this PR do?

• Fixes Add mobile FAB buttons to Calendars and Conferencing settings pages #26406
• Fixes CAL-7010

Visual Demo (For contributors especially)

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Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. If N/A, write N/A here and check the checkbox.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

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Summary by cubic

Changed the “Add Calendar” and “Add Conferencing” actions to small floating action buttons (FAB) on mobile to make them easier to find and tap. Implements Linear CAL-7010 by setting size=sm and variant=fab in the Calendars and Conferencing settings.

^{Written for commit 7ae3852. Summary will update on new commits.}

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[cubic-dev-ai]

No issues found across 2 files

[dhairyashiil]

Using fab variants here doesn’t seem necessary, so I think current approach is fine as-is.