feat: design system overhaul — sidebar, AI chats, settings, brainstorm, color cleanup

- Sidebar: dynamic brand-accent colors, brainstorm section restyled
- AI chat general: popup panel with expand/collapse, hides when contextual AI open
- AI chat contextual: tabs reordered (Actions first), X close button, height fix
- Settings: all tabs restyled, 6 new color presets (sage, terracotta, iron, etc.)
- Global color cleanup: emerald/orange hardcoded → brand-accent dynamic
- Brainstorm page: orange → brand-accent throughout
- PageEntry animation component added to key pages
- Floating AI button: bg-brand-accent instead of hardcoded black
- i18n: all 15 locales updated with new AI/billing keys
- Billing: freemium quota tracking, BYOK, stripe subscription scaffolding
- Admin: integrated into new design
- AGENTS.md + CLAUDE.md project rules added

memento-note/app/api/brainstorm/[sessionId]/expand/route.ts

@@ -2,10 +2,15 @@ import { NextRequest, NextResponse } from 'next/server'
 import prisma from '@/lib/prisma'
 import { auth } from '@/auth'
 import { z } from 'zod'
-import { getTagsProvider } from '@/lib/ai/factory'
+import { runLaneWithBillingUser, willUseByokForLane } from '@/lib/ai/provider-for-user'
 import { getSystemConfig } from '@/lib/config'
 import { embeddingService } from '@/lib/ai/services/embedding.service'
 import {
+  checkSessionEntitlementOrThrow,
+  QuotaExceededError,
+} from '@/lib/entitlements'
+import {
+  billingOwnerFromSession,
   verifyParticipant,
   resolveAiContextUserId,
   sanitizeNotesForGuest,
@@ -182,7 +187,23 @@ export async function POST(
       return NextResponse.json({ error: 'Session not found' }, { status: 404 })
     }
 
-    const parentIdea = brainstormSession.ideas.find(i => i.id === ideaId)
+    const { billingOwnerId, isGuestActor } = billingOwnerFromSession(
+      brainstormSession.userId,
+      session.user.id,
+    )
+    // Story 3.5: per-provider BYOK bypass
+    const earlyConfig = await getSystemConfig()
+    const { usedByok: willUseByok } = await willUseByokForLane('tags', earlyConfig, billingOwnerId)
+    if (!willUseByok) {
+      await checkSessionEntitlementOrThrow(
+        billingOwnerId,
+        session.user.id,
+        isGuestActor,
+        'brainstorm_expand',
+      )
+    }
+
+    const parentIdea = (brainstormSession.ideas || []).find(i => i.id === ideaId)
     if (!parentIdea) {
       return NextResponse.json({ error: 'Idea not found' }, { status: 404 })
     }
@@ -205,7 +226,6 @@ export async function POST(
     }
 
     const config = await getSystemConfig()
-    const provider = getTagsProvider(config)
 
     const prompt = buildExpandPromptV2(
       parentIdea.title,
@@ -216,7 +236,12 @@ export async function POST(
       locale
     )
 
-    const llmResponse = await provider.generateText(prompt)
+    const { result: llmResponse } = await runLaneWithBillingUser(
+      'tags',
+      config,
+      billingOwnerId,
+      (provider) => provider.generateText(prompt),
+    )
 
     let newIdeas: any[]
     try {
@@ -311,6 +336,9 @@ export async function POST(
 
     return NextResponse.json({ success: true, data: updatedSession })
   } catch (error: any) {
+    if (error instanceof QuotaExceededError) {
+      return NextResponse.json(error.toJSON(), { status: 402 })
+    }
     if (error instanceof z.ZodError) {
       return NextResponse.json({ error: error.issues }, { status: 400 })
     }