[2.x] feat: setting to force a FontAwesome icon style forum-wide

[imorland]

Changes proposed in this pull request

Adds a Forced Icon Style setting (fontawesome_forced_style, default empty = zero behavior change) to the FontAwesome section of the admin Advanced page. When set — e.g. fa-duotone fa-light, or simply fa-regular — every icon rendered through the Icon component swaps its declared style classes for the configured one, giving forums with FontAwesome Pro a uniform icon style without forking extensions.

• Recognition is a closed style-class list, both syntaxes per family (fas/fa-solid, far/fa-regular, light/thin/duotone, the sharp variants, bare fa). Deliberately not pattern-based: icon names (fa-lightbulb) and utility classes (fa-fw, fa-spin, fa-2x) also start with fa- and pass through untouched.
• Brand icons are never rewritten — their glyphs only exist in the brands family; forcing a style would blank every GitHub/Discord icon.
• noStyleOverride prop on Icon is the per-icon escape hatch, for icons whose style is their meaning (the solid vs regular star conveying starred state). Stripped before the DOM spread, pinned by test.
• The forum attribute only ships when the feature is on (visible() guard), and non-string icon names pass through instead of throwing mid-render (an Icon without a name previously took down the whole mithril tree during live testing — now pinned).
• tags' tagIcon helper renders through Icon — previously it built <i> directly, so the tag sidebar (the most visible icon surface on most forums) bypassed the setting entirely.
• Input.less fix found during live testing: FA's duotone CSS sets position: relative on the icon element itself (for its two-glyph ::after layering) and loads after our stylesheet at equal specificity — popping the search box's prefix icon out of the input into normal flow. Pinning position: absolute !important follows the exact pattern that block already documents for FA Kit's injected width/display styles, and duotone renders correctly on any non-static position.

The style must be available in the forum's configured icon source — most styles need FontAwesome Pro; the setting's help text says so.

Reviewers should focus on

• The style-class list in applyIconStyle.ts (22 table-driven unit tests: every family in both syntaxes, sharp compounds replaced atomically, brands exemption, fa-lightbulb-style false positives, utility passthrough, duplicate-style collapse, non-string names).
• Icon component tests: set/unset behavior, brands, noStyleOverride (including that it doesn't leak as a DOM attribute).
• Verified live with fa-duotone fa-light and fa-light: 117/122 icons rewritten on the index page, brands intact, clean console, search box intact.

Confirmed

• Frontend changes: tests are green (yarn jest), typings check passes.
• Backend changes: tests are green (run composer test).