fix: chat memory lost between messages + per-note history

Chat (AIChat floating widget): conversationId was never captured from
the API response, so every message created a new conversation with no
context. Now creates the conversation upfront before streaming (same
pattern as ChatContainer) so the ID persists across messages.

Note history: was stored globally in UserAISettings, so enabling
history on one note enabled it for ALL notes. Now each Note has its
own historyEnabled boolean field. The "Enable history" action only
affects the specific note. A migration adds the column with default
false.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

memento-note/app/api/notes/[id]/route.ts

@@ -142,7 +142,7 @@ export async function PUT(
     })
 
     try {
-      const historyEnabled = await isNoteHistoryEnabledForUser(session.user.id)
+      const historyEnabled = existingNote.historyEnabled === true
       if (historyEnabled && shouldCaptureHistorySnapshot(updateData)) {
         const mode = await getNoteHistoryMode(session.user.id)
         if (mode === 'auto') {