feat(mosaic): refine UserButton popover width, header, and settings label

[prestonwebdev]

Description

Stacked on top of #10002 (carp/mosaic-user-button-combined-settings) — a set of small design refinements to the Mosaic UserButton, split out so they can be reviewed and merged into that PR independently if wanted.

• Popover width → 350px. The open surface now uses a fixed 350px width (clamped to the viewport) instead of the default popover md variant (~376px).
• Header avatar → 38px. The selected account/organization avatar in the open menu is now 38px (up from 32px). 38px maps to the shared Avatar md size, so the non-badged avatar uses it directly.
• Header name + identifier both 14px. The identifier/subtitle line moves from 12px to 14px so it matches the name's size — the two are now distinguished only by weight (name medium, identifier regular).
• User-led "Settings" button. In user-led mode the header settings control now renders as a labelled button with a gear icon and the word "Settings", rather than the icon-only gear. Other modes are unchanged.

Checklist

• pnpm test runs as expected.
• pnpm build runs as expected.
• (If applicable) JSDoc comments have been added or updated for any package exports
• (If applicable) Documentation has been updated

Type of change

• 🐛 Bug fix
• 🌟 New feature
• 🔨 Breaking change
• 📖 Refactoring / dependency upgrade / documentation
• other: Design refinements to the (unreleased) UserButton rework in feat(mosaic): rework UserButton #10002

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🦋 Changeset detected

Latest commit: 6fe2c6b

The changes in this PR will be included in the next version bump.

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