Bump Angular to 21 lts

@angular/core, @angular/common and @angular/compiler still carry advisories; every fix for them lands in Angular ≥ 18.2.15 / ≥ 19.2.26. Dependabot's proposal (#859, #981) is to jump to v21-lts, which cannot be done without UI changes:

src/@treo/styles/treo.scss, src/@treo/styles/vendors/_angular-material.scss and src/@treo/styles/utilities/_colors.scss are built on the legacy Angular Material Sass API (@import '~@angular/material/theming', mat-core(), mat-light-theme($p, $a, $w)). That API was removed in Angular Material 15 — the same release that moved every component to MDC, which changes component metrics and appearance and orphans the .mat-form-field-* overrides in src/@treo/styles/overrides/_angular-material.scss. Doing that upgrade properly means rewriting the Treo theming layer.

Might put off until a 1.0.0 rewrite to react, svelte, etc as the vulnerabilities are mostly XSS which is not as severe for scrutiny.