feat(web): whole-page image drop, projected intake limits, and thumbnail tiling

Second alignment step for #2248: document-level drag intake behind the new
DropOverlay atom, lightbox close icon and shared dialog mask, DeepSeek Chat
thumbnail rules (single 240px long edge with ratio clamp, 64px tiles, merged
consecutive assistant images), image limits raised to 20/10MiB/100MiB and
published to clients as the imageLimits projection, whole-batch intake
pre-check with product-copy banners, and attachment-error reasons mapped to
localized copy.

packages/host/apiproxy/tests/api-proxy-projections.spec.ts

@@ -11,6 +11,7 @@ import { describe, expect, it } from 'vitest'
 import { Context } from '@deepseek-ai/cordis'
 import { z } from 'zod'
 import AgentRegistry, { Inbox } from '@deepseek-ai/dsh-agent'
+import { AttachmentStore } from '@deepseek-ai/dsh-attachment'
 import type { Agent } from '@deepseek-ai/dsh-agent'
 import { createUserMessage } from '@deepseek-ai/dsh-llm'
 import SessionStore, { SessionId } from '@deepseek-ai/dsh-session'
@@ -86,6 +87,51 @@ describe('session.history projections block', () => {
     expect(events.at(-1)?.event.seq).toBe(projections?.asOfSeq)
   })
 
+  it('publishes the attachments imageLimits as a constant unit while both seams are composed', async () => {
+    const { ctx, session } = await harness(true)
+    const limits = {
+      maxImageBytes: 10 * 1024 * 1024,
+      maxImagesPerMessage: 20,
+      maxMessageImageBytes: 100 * 1024 * 1024,
+      maxImagePixels: 40_000_000,
+      mediaTypes: ['image/png'] as const,
+    }
+    await ctx.plugin(class extends AttachmentStore {
+      readonly imageLimits = limits
+      validateImage(): Promise<void> { return Promise.resolve() }
+      saveImage(): Promise<never> { return Promise.reject(new Error('unused')) }
+      readImage(): Promise<never> { return Promise.reject(new Error('unused')) }
+    })
+    const gateway = api(ctx)
+    seedMessages(session, 2)
+    const response = await gateway.sessions.history(request({ sessionId: session.id }))
+    if (!response.result.ok) throw new Error('history failed')
+    expect(response.result.value.projections?.values['imageLimits']).toEqual(limits)
+    // Constant unit: appending events must never broadcast an imageLimits frame.
+    await new Promise(resolve => setTimeout(resolve, 0))
+    const abort = new AbortController()
+    const stream = gateway.events.mux({ rpcId: RpcId('t-limits-mux'), payload: {} }, abort.signal)
+    const frames: MuxFrame[] = []
+    const drained = (async () => {
+      for await (const envelope of stream) {
+        frames.push(envelope.payload)
+        if (frames.some(f => f.type === 'session/event')) abort.abort()
+      }
+    })().catch(() => {})
+    seedMessages(session, 1)
+    await drained
+    expect(frames.some(f => f.type === 'session/projection' && f.key === 'imageLimits')).toBe(false)
+  })
+
+  it('leaves the imageLimits key absent while no attachment service is composed', async () => {
+    const { ctx, session } = await harness(true)
+    seedMessages(session, 1)
+    const response = await api(ctx).sessions.history(request({ sessionId: session.id }))
+    if (!response.result.ok) throw new Error('history failed')
+    expect(response.result.value.projections).toBeDefined()
+    expect('imageLimits' in (response.result.value.projections?.values ?? {})).toBe(false)
+  })
+
   it('never carries the block on loadOlder pages (beforeSeq present)', async () => {
     const { ctx, session } = await harness(true)
     ctx.sessionProjections.register(lastUserUnit())