feat: redesign Cloud settings UI and switch to OAuth redirect login

- Move Cloud section inline with other settings, remove dedicated header bar
- Replace in-extension login form with browser redirect to accounts.betterseqta.org
- Background script intercepts OAuth callback URL to capture tokens
- Add animated CloudPanel overlay (same pattern as ColourPicker)
- Hide cloud sync details and profile picture setting when not signed in
- Simplify CloudSettingsSync UI, reduce text verbosity
- Fix settings download to merge keys instead of clear+set
- Add legacy-to-plugin settings migration for cloud sync
- Shorten profile picture and default page descriptions
- Make DisclaimerModal title/message dynamic
- Update CloudHeader button styling to match other buttons

src/plugins/built-in/profilePicture/index.ts

@@ -14,8 +14,7 @@ const settings = defineSettings({
   useCloudPfp: booleanSetting({
     default: false,
     title: "Use BetterSEQTA Cloud profile picture",
-    description:
-      "When enabled, uses the avatar from your BetterSEQTA Cloud account (sign in from the extension store). Otherwise uses the uploaded image below.",
+    description: "Use your cloud account avatar instead of the uploaded image below",
   }),
   picture: componentSetting({
     title: "Profile Picture",