Account Switching Experience: `AccountSwitcher` UI, multi-tab focus, and "Add account" flow

[saknarajapakshe]

Goal

Let users see all their linked accounts, switch between them, and add new ones — via a menu on the profile avatar.

How switching works

Clicking any linked account opens or focuses a tab for it, never swapping the tab you're already in. Adding a new account works the same way: once it's added, a button opens it in a new tab, leaving your current tab untouched.

Every new piece of UI here should follow the app's existing responsive pattern — full-screen dialogs on mobile, regular centered dialogs on desktop (same as ComposeDialog today).

Sub-issues

• Multi-tab plumbing in AccountContext + reusable ConfirmDialog #33 — Account context/tab plumbing + a reusable confirm dialog
• AddAccountForm: two-step "Add another account" form component #34 — Two-step "add account" form component
• Build AddAccountModal #35 — Build the "Add account model" using that form
• AccountSwitcher menu + Header integration #39 — The switcher menu itself + hooking it into the header

Build order: #33 and #34 can start in parallel today. Then #35. #39 goes last — it also needs #36 and #37 from #25.

Depends on

#28 (AccountContext).