feat: dark mode + MUI component migration

[ShyamThangaraj]

Summary

Adds full dark mode support to PeterPlate and cleans up the codebase by replacing hardcoded HTML elements with proper MUI components. Used MUI's built-in theming where possible, and fell back to Tailwind dark: variants where MUI didn't play nice. Also fixed some pre-existing light mode color issues along the way. Theme switching is handled by next-themes.

Changes

• Set up the MUI dark palette and updated the dark mode CSS variables to match the Figma design
• Replaced hardcoded text and layout elements with MUI Typography and Box components across the codebase, using semantic color props that automatically respond to the active theme
• Fixed light mode color inconsistencies across the toolbar, sidebar, restaurant filters, cards, and dialogs
• Updated interactive elements (dropdowns, meal selector, station tabs, view toggles, buttons, badges) to use the correct colors in both modes

Additionally closes #656

Testing Instructions

1. nvm use 20 && docker compose up -d && pnpm run dev
2. Seed: pnpm --filter @peterplate/db db:migrate && cd apps/server && pnpm run test:weekly
3. Toggle Light/Device/Dark via the profile sidebar and check the home page, restaurant pages, events, about, and any dialogs