Add "Reset settings to defaults" button

[gek0z]

Summary

Add a button in Settings to reset all user preferences back to their default values in one click. Useful after fiddling with toggles or when troubleshooting.

Proposed UI

A new row in the existing final Section (next to "Quit YouMenuTube"), styled as a destructive button with a confirmation dialog before it acts.

[ Reset settings to defaults ]   (destructive, shows confirmation)

Scope

Reset the following @AppStorage keys referenced by SettingsView.swift:

• player.autoplay → true
• player.floatOnTop → true
• home.hideShorts → true
• subscriptions.hideShorts → true
• playlists.pinnedId → ""
• playlists.pinnedTitle → ""

Out of scope (intentionally)

• Session / cookies — already handled by "Sign out" in the Account section; a preferences reset shouldn't log the user out.
• Launch-at-login — managed by SMAppService, not UserDefaults; leave the toggle where the user last set it.
• Update checker state — transient, not user-configurable.

Acceptance

• Button lives in Settings, visually separated from destructive actions that affect data (Sign out, Quit).
• Confirmation dialog before wiping.
• After reset, all toggles in Settings reflect their defaults without requiring a relaunch.