feat(portal): add theme switcher with system preference support

[BatLeDev]

Add a theme switcher to the portal nav bars and support a system theme that follows the user's OS color-scheme / contrast preferences.

• Mount the shared theme-switcher (from @data-fair/lib-vuetify) in both layout-nav-bar and layout-personal-app-bar.
• Replace the inline theme-cookie logic in 03-vuetify.ts with a resolvePortalTheme helper: the theme cookie now defaults to system, which resolves to default/dark/hc/hc-dark from prefers-color-scheme (and forced-colors), gated on the theme variants the portal enables.
• Read the preference from vuetify-nuxt-module's SSR client hint on the server and matchMedia on the client; enable the prefersColorScheme SSR client hint in nuxt.config.ts.
• Add @data-fair/lib-vue as a relative dependency for the Theme/AppliedTheme types.

Why: part of the cross-repo theme-switcher feature (lib + portals + data-fair); the portal needs an in-UI control and a system mode that respects OS preferences.

Regression risks:

• Depends on unreleased @data-fair/lib changes — the theme-switcher.vue component, the Theme/AppliedTheme exports in lib-vue/session, and the system cookie value. The portal can't build/CI-pass until that lib version is published and pinned; the relative dep currently points at a local checkout.
• Default behavior changes: the theme cookie default moves from default to system, and Vuetify's defaultTheme is now the resolved theme. Existing portals that configured a dark/hc variant (and enabled the corresponding dark/hc/hcDark flag) will now auto-serve it to users whose OS prefers dark/high-contrast, where before everyone got the light default.
• First-load flash / hydration: before the SSR client hint is known, the server may resolve to light while the client's matchMedia resolves to dark. Worth checking first-paint behavior across browsers (note reloadOnFirstRequest is already disabled as broken in Brave).
• Cookie contract with the lib switcher: only the single resolved theme is registered in vuetifyOptions.theme.themes, so live switching relies on the lib's theme-switcher writing the theme cookie in Theme shape and reloading — verify a switch actually re-renders.