Fix broken icons across the site

[djoufson]

Summary

• Add the missing Font Awesome stylesheet — fa-solid, fa-regular and fa-brands icons are used across Contact, Events, Login, Jobs, Error and the .NET Conf pages but no CSS was ever loaded for them.
• Properly escape the second @ in the Phosphor CDN URLs (web@@2.1.1) and switch to jsdelivr so the version segment is reliably preserved through Razor parsing.
• Scope the default font-family / font-weight rule to text elements (body, p, span, ul, li, h1..h4) instead of *, ::before, ::after, .... The wildcard rule was setting font-family: Lexend directly on ::before pseudo-elements, which overrode the icon fonts ("Phosphor", "Font Awesome ...") on the very pseudo-element that renders the glyph — so icons rendered as tofu boxes.
• Same scoping for the default text-dark-purple color rule, so icon colors can inherit from their parent (e.g. <a class="text-primary"><i class="ph ph-..."></i></a> now works).

Test plan

• Hard-refresh the home page — Phosphor icons (arrows, whatsapp, github, globe, hearts, etc.) render correctly
• Contact page — fa-brands (discord, linkedin, github, whatsapp, telegram) render
• Jobs pages — fa-solid / fa-brands icons render
• Error and .NET Conf 2024/2025 pages — FontAwesome icons render
• Icons inside text-primary / text-white / etc. containers inherit the parent color

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