[Feat][UI] Account Settings Panel Component

[saknarajapakshe]

Summary

Adds the "Manage your Account" dialog - a settings popup for your profile and theme, opened from the avatar icon in the header (instead of navigating to /profile).

• Profile tab - edit your display name and save it, see your email (read-only, since it's tied to the account's mail login), and remove the account if you need to.
• Appearance tab - switch between light/dark theme. This replaces the toggle button that used to sit in the header.
• Removing an account now confirms first with a reusable ConfirmDialog before actually calling removeAccount().
• No security/password tab for now - there's no backend for that yet, so it's better to leave it out than fake it.

Screenshots

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Closes #37

Test plan

• typecheck passes
• build passes
• clicked through both tabs, edited the name and saved, switched theme and reloaded to confirm it stuck, and removed an account