AccountSwitcher menu + Header integration

[saknarajapakshe]

Part of: #23

What to build

The actual switcher menu, opened from the avatar in the header:

• Shows the current account and its unread count.
• "Manage your Account" → Account Settings Panel (Profile / Appearance tabs) #37's dialog.
• A collapsible list of other linked accounts — clicking one opens or focuses its own tab.
• "Add another account" → Build AddAccountModal #35's dialog.
• "Sign out of all accounts" → confirm → Implement removeAccount() and logoutAll() in AccountContext #36's logoutAll() → back to the login screen.

This replaces the current avatar click, which just navigates to /profile.

Done when

• All linked accounts show up, with the active one clearly marked.
• Clicking an account never opens a duplicate tab for it.
• Works well on both desktop and mobile.
• "Sign out of all accounts" always confirms first.

Depends on

#33, #35 (this issue) and #36, #37 (#25) — this is the one place in the project that needs both halves finished first.