refactor: Upgrade app to Nuxt 3

[Hanziness]

This PR introduces a huge refactoring of the app as the underlying engine is upgraded from Nuxt 2 to Nuxt 3. The most noteworthy changes (most of these are not user-facing):

• ⚠️ The app is now managed using pnpm instead of yarn. Please delete the previous node_modules folder, and reinstall using pnpm install --shamefully-hoist (this was reverted due to incompatibilities will full plug-and-play support and Netlify not supporting pnpm, see chore(dev): Revert package manager to yarn #257)
• ⚠️ The settings store had a minor, but breaking, change (in how colours are stored), so the settings (but not the tasks) will be reset. For those who are using the develop branch: while the version in package.json is still on 1.2.0 the settings will reset on every page reload - this can be temporarily worked around by manually changing the version field in package.json to 1.3.0. This issue will automatically go away when 1.3.0 is released.
• Layout transitions are temporarily disabled due to visual glitches (could be a Nuxt bug)
• A bunch of dependencies have been removed, like Nuxt's PWA module (nuxt/pwa) (due to incompatibilities), thenuxt/image module and the nuxt/i18n module.
• A proper range input has been introduced for numeric entries
• The time input has also changed - it is experimental. You can switch between the minutes and seconds entry using the buttons next to the text input
• Stores, plugins and some components have been moved over to TypeScript and refactored
• Modules that were entirely rewritten have been converted to use Vue 3's Composition API and TypeScript. For now, some components are still using the old Options API. I plan to convert more components to the Composition API in the future as it provides much better type hinting.
• Totally revamped settings controls which should be much easier to maintain
• The ticker and notification controller was moved into a composable (they are not part of the DOM anymore) -- the notification controller will be later transformed into the web platform module (there will be a "mobile" module, too, stay tuned)
• The PWA manifest and service worker are now manually generated, so the app can use the latest version of Workbox (most importantly v6 instead of v5), and proper offline pre-caching. Previously the service worker only cached items (like the settings panel or languages) that were loaded before going offline, now the service worker can download everything for offline use - so you can pretty much go offline after loading the page and stuff should just work!
• The translations module is also manually integrated, making it easier to keep up-to-date.

Phew.

Closes #248