[gnacho]

Closes #318

This moves the Add Folder shortcut into the main window hamburger menu, between Add Account and Preferences. The action follows the selected account and is disabled when no account is available. The button above the folder list is gone.

The shared Add Folder dialog now shows Account: user (server) before the folder fields. The same account line remains visible when validation reopens the dialog. Existing translations cover the new UI, and Preferences remains unchanged.

Tests cover the menu action, its disabled state without accounts, active account selection with A/B fixtures, removal of the old button, and the destination account label.

Local checks:

• cargo build
• cargo test: 769 passed, 0 failed, 3 ignored
• cargo clippy --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check
• i18n catalog parity: 538 entries, no diff