feat(theme): add Nexus — AAA-grade UI/UX theme alongside DefaultRevamp

[myream2003]

Summary

Introduces Nexus, a complete new public-frontend theme that ships alongside the existing DefaultRevamp. DefaultRevamp, the core, all modules, and the panel templates are untouched — this PR is purely additive.

Nexus is dark-first with optional light mode, built on TailwindCSS + Alpine.js, and coexists with the legacy jQuery / Fomantic UI stack that modules rely on. Design language is in the Linear / Vercel / Discord / Notion / Raycast direction.

What's in the box

• Theme bootstrap: template.php, template_settings/{settings.php,settings.tpl}, package.json, tailwind.config.js, postcss.config.js
• Design system: src/styles/tokens.css (CSS-variable tokens), base.css, components.css (.nx-* component library), overrides.css (legacy compatibility layer that restyles .ui.button, .ui.segment, .ui.form, .ui.message, .ui.modal, .ui.grid, DataTables, Select2, etc. so module-emitted markup looks consistent)
• Alpine bundle (src/scripts/main.js): theme / toast / modal / mobileNav / palette stores, Cmd+K command palette, x-tooltip & x-clipboard directives, global escape handling, jQuery coexistence
• Layout: new header.tpl, glassmorphic sticky navbar.tpl with off-canvas mobile drawer, footer.tpl
• All public-frontend pages re-implemented: landing, portal, auth flow (login/register/forgot/change/tfa/complete-signup/authme/registration-disabled/user-not-exist), profile (banner, tabs, wall posts with reactions, replies, edit/delete/block/banner modals), user CP (index, settings, navigation + baseline copies for the rest), forum (index/view-forum/view-topic/new-topic/edit/search/follow/merge/move/redirect-confirm/no-discussions), members directory, leaderboards, status, 403/404/maintenance, cookies/privacy/terms, custom, reactions modal, user popover
• Widgets: online_staff, online_users, server_status, statistics, minecraft_account (with skinview3d), profile_posts, reactions, cookie_notice, widget_error, forum/latest_posts
• Docs: README.md (install + build + activate + customize + smoke-test + troubleshooting), docs/CHANGES.md (file-by-file inventory)

Why

DefaultRevamp's Fomantic UI design is functional but dated. This PR provides a modern alternative without forcing anyone off the existing theme. Operators can switch between Nexus and DefaultRevamp from StaffCP → Layout → Templates at any time.

Crucially, the Smarty-variable contract is preserved. Every page consumes the same \$NAV_LINKS, \$USER_SECTION, \$WIDGETS_*, \$REACTIONS_*, \$FORUMS, \$REPLIES, etc. that DefaultRevamp does, so all controllers and module integrations keep working.

Not in this PR

• Admin panel (custom/panel_templates/) — slated for a follow-up.
• Email templates and deep TinyMCE skinning — slated for a follow-up.

Reviewer checklist

• custom/templates/Nexus/ is fully self-contained
• custom/templates/DefaultRevamp/, core/, modules/, and custom/panel_templates/ have zero diffs in this PR
• Build artifacts (assets/css/*.css, assets/js/*.js, node_modules/) are gitignored
• Smarty template safety: no PHP static method calls inside .tpl files, no unsupported \{ brace escapes, every {include file=…} target exists, every {\$smarty.foreach.X.…} has a matching name=X declaration

Local testing

cd custom/templates/Nexus
npm install
npm run build

Then StaffCP → Layout → Templates → activate Nexus. Walk the smoke-test plan in custom/templates/Nexus/README.md §6.

Test plan

• Visit /, /forum, /profile/<user>, /login, /register on desktop + mobile — no console errors, theme toggle persists across reloads
• Create / reply / react / edit / delete a forum topic
• Edit profile + change avatar + post on wall + react
• Cmd/Ctrl+K opens command palette, Escape closes overlays, off-canvas drawer works on mobile
• Switch back to DefaultRevamp — everything still works (parallel-theme guarantee)
• Lighthouse on local install: Performance ≥ 90, A11y ≥ 95