AddAccountModal + AccountSwitcher: add and switch between accounts

[saknarajapakshe]

What this does

• Adds AddAccountModal - the two-step "add another account" form now lives in a proper dialog: icon + title + close button up top, filled-style inputs, the dots moved below the fields, and a back arrow instead of a separate "Back" button. Finishing step 2 opens the new account in its own tab and closes the modal.
• Adds AccountSwitcher and wires it into Header.tsx - clicking the avatar in the header now opens a real switcher menu (current account + unread count, "Manage your Account", other linked accounts, "Add another account", "Sign out of all accounts") instead of just navigating to /profile.

Stacked on #48 (and transitively #46, #45) - this diff includes their commits until those land.

Closes #35
Closes #39

Test plan

• typecheck passes
• build passes
• manually click through: add an account (both steps, back arrow, dots), switch to another linked account, open "Manage your Account", sign out of all accounts