Add dark-mode support across the frontend

[SSpirate11]

Fixes: #591 , #22

Adds a site-wide dark mode to the Montage frontend. A theme toggle in the
header lets users switch between light and dark, with their choice persisted
across sessions. All major views are restyled to use CSS theme variables so
they render correctly in both themes.

Changes done :

• Theme store: a Pinia store that tracks isDark,
  exposes init()/toggle(), persists the choice to localStorage
  (_montageTheme), and applies it via a data-theme attribute on .
• Theme variables (assets/main.css): light/dark CSS custom properties
  (backgrounds, surfaces, text, links, borders) and dark-mode style rules.
• Header toggle (AppHeader.vue): a sun/night switch wired to the theme
  store, plus header colors moved onto theme variables.
• App root (App.vue): initializes the theme store on startup.
• Component restyling: hardcoded colors replaced with theme variables
  across Campaign, Round, and Vote components, the footer, login box,
  avatar, and permission-denied view.

Working

The selected theme is stored in localStorage and reflected as
data-theme="dark|light" on the document root. CSS variables resolve per
theme, so components stay theme-agnostic. Theme defaults to light on first
visit.

Screencast.from.2026-06-27.15-38-35.online-video-cutter.com.mp4