Organization UI — members, settings, and org switcher

[jeroenrinzema]

Goal

Build the frontend UI for managing organizations — currently organizations exist in the database but have no UI representation.

Depends on: #44, invite system backend

Scope

Organization Switcher

• Add an org switcher to the navigation/sidebar so users can switch between organizations they belong to.

Organization Settings Page

• Organization details (name, etc.).
• Member list — show all members of the organization with their roles.
• Ability to change member roles or remove members.

Organization Invite Flow

• Invite dialog — enter an email address and select a role to send an org invite.
• Show pending invites with the ability to revoke them.