Use platform detection to render OS-specific install instructions in UserDropdown #28252

Background

In #28244, we introduced platform detection helpers (isMac(), isWindows(), supportsCoderDesktop()) in site/src/utils/platform.ts to gate the visibility of the new Install Coder Desktop item in UserDropdown based on the user's OS.

During review, @matifali pointed out that this same platform detection could be leveraged to render an OS-specific install script/command elsewhere in the UI, rather than always showing generic instructions.

Note for later: Wow, this can also be used to render the OS Specfic install script.

Proposal

Investigate where in the product we currently show install commands/scripts (for the Coder CLI, and potentially Coder Desktop) and where we could tailor them based on the detected platform. For example:

• Show the appropriate curl | sh / Homebrew / WinGet / PowerShell command based on isMac() / isWindows() / Linux.
• Fallback to a platform picker (or all options) when detection is ambiguous.

Scope / Open questions

• Which surfaces should get OS-aware install instructions? (Install CLI modal, Coder Desktop docs link, onboarding, etc.)
• Do we want a shared component for rendering platform-aware install snippets?
• How do we handle iPadOS (which reports as macOS) and other edge cases already handled in supportsCoderDesktop()?

References

• PR: feat(site): add Install Coder Desktop item to UserDropdown #28244
• Review comment: feat(site): add Install Coder Desktop item to UserDropdown #28244 (comment)
• Helper module: site/src/utils/platform.ts

Created on behalf of @jakehwll