feat(email-list): add bulk Not-Spam action to selection toolbar in junk

[shukiv]

What

Adds a Not spam button to the bulk-selection toolbar, shown only when viewing a junk folder — alongside the existing mark-read / mark-unread / delete actions. Selecting spam messages and clicking it un-junks them in one shot, instead of having to open the context menu.

Why

The batch un-spam action already exists (onBatchUndoSpam in the context menu); this just surfaces it as a first-class toolbar icon where it's most useful. Matches the discoverability of the other bulk actions.

Implementation

• New handleBatchUndoSpam handler in email-list.tsx, reusing the existing batchUndoSpam store action and the same success/error toasts as the context-menu path.
• Junk-only toolbar button (ShieldCheck icon), gated on effectiveMailboxRole === 'junk' (works for the aggregate "All Junk" view too).
• Reuses the existing context_menu.not_spam label — no new i18n keys (already translated in all locales).

Testing

• In Junk: select messages → Not spam appears → click → messages leave Junk, success toast
• In Inbox/other folders: button not shown
• tsc clean; lint clean (no new warnings)