FEAT GUI: Preserve chat drafts when exploring datasets

Start with frontend\src\App.tsx, components\Chat\ChatWindow.tsx, ChatInputArea.tsx, ChatInputAreaHandle.restoreDraft/getDraftRevision, hooks\useChatConverters.ts, and the route helpers, following the frontend style and test instructions. Verify the ordinary, unsaved, rapid-navigation, stale-completion, and attachment-cleanup cases described in the acceptance criteria, with a single safe pending handoff on return.

Is your feature request related to a problem? Please describe.

Chat draft text, attachments, and converter state currently live inside chat components. Navigating to Datasets must not discard that work or lose which attack/conversation the user intended to return to.

Work item 6 of 17 in #2744. This prepares the draft handoff used by items 7-8 and remains not ready yet until promoted after the preceding work item.

Describe the solution you'd like

Add a typed, navigation-safe draft/context handoff using the existing React state/context conventions. Keep state at the narrowest shared owner needed for the chat-to-dataset journey; do not introduce another state-management library.

Preserve the originating attack and active conversation, text, attachments, system-prompt draft, selected target, and converter configuration/applied draft state. Distinguish a new unsaved chat from a persisted attack. Preserve dataset filters/page/selection for returning to the explorer.

Provide an explicit pending-import payload that can be applied once after the destination chat is ready. Guard against stale asynchronous navigation/import results overwriting a newer draft. Avoid timeout-based synchronization and make attachment/blob URL ownership and cleanup explicit.

Acceptance criteria:

• Visiting Datasets and returning preserves the existing draft and originating conversation.
• Text plus media and converter/system-prompt state survive the navigation round trip.
• A pending handoff is applied once, not duplicated by rerenders or Back/Forward navigation.
• Switching context or encountering a missing/locked destination does not overwrite an unrelated draft.
• Focus and dirty-draft replacement can be managed by the import UI.
• Tests cover ordinary return, unsaved chat, rapid navigation, stale completion, and attachment cleanup.

Describe alternatives you've considered, if relevant

Do not persist GUI drafts as conversation history, place prompt/media content or credentials in URLs, add a memory schema, or build a batch draft queue. Full cross-session draft persistence is not required by this issue.

Additional context

Start with frontend\src\App.tsx, components\Chat\ChatWindow.tsx, ChatInputArea.tsx and ChatInputAreaHandle.restoreDraft/getDraftRevision, hooks\useChatConverters.ts, and the route helpers. Preserve existing chat recovery and target-resolution behavior. Follow frontend style/test instructions.