client/web: ensure path prefix has a leading slash

This is simply an extra check to prevent hypothetical issues if a prefix
such as `--prefix="javascript:alert(1)"` was provided.  This isn't
really necessary since the prefix is a configuration flag provided by
the device owner, not user input.  But it does enforce that we are
always interpreting the provided value as a path relative to the root.

Fixes: tailscale/corp#16268

Signed-off-by: Will Norris <will@tailscale.com>
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