feat(mobile): large + multi-image uploads from the camera-roll picker

The mobile copy/paste overlay's "🖼 Image" button (and drag-drop / paste)
now handles real-world photo batches:

- Up to 20 images per batch, uploaded with bounded concurrency (3) and a
  live "Uploading N/M…" progress toast; a final summary reports successes,
  any failures, and whether the 20-cap trimmed the selection (no silent
  truncation).
- Per-file upload limit raised 10MB → 50MB (MAX_PASTE_IMAGE_BYTES in
  buffer-limits.ts, env-overridable) so full-resolution phone photos and
  large screenshots aren't rejected.
- Very large images are downscaled to <=4096px longest edge before upload:
  fixes iOS Safari's ~16.7M-px <canvas> limit (which made huge photos fail
  to re-encode and fall back to an original that tripped the magic-byte
  check), and keeps batch uploads fast and small.
- Fix a latent concurrency bug the batch path exposed: the first parallel
  uploads to a session raced on `mkdir(.claude-images)` and the EEXIST
  losers 500'd. mkdir now treats an existing real directory as success
  (re-verifying it isn't a planted symlink), so concurrent uploads succeed.

Verified end-to-end in a real browser (Playwright): downscale, >10MB
server acceptance, 20-cap, 20/20 concurrent uploads landing on disk.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/web/server.ts

@@ -128,6 +128,7 @@ import {
 } from '../utils/index.js';
 import type { EventLoopMonitorHandle } from '../utils/index.js';
 import { MAX_CONCURRENT_SESSIONS, MAX_SSE_CLIENTS } from '../config/map-limits.js';
+import { MAX_PASTE_IMAGE_BYTES } from '../config/buffer-limits.js';
 import { SseEvent } from './sse-events.js';
 import { getLatestPlanUsage } from './plan-usage-latest.js';
 import type { ScheduledRun } from './ports/index.js';
@@ -679,8 +680,8 @@ export class WebServer extends EventEmitter {
     // last byte (hard-coded \r\n offsets), and there was no part-count cap.
     await this.app.register(fastifyMultipart, {
       limits: {
-        fileSize: 10 * 1024 * 1024, // 10MB per file
-        files: 1, // paste-image only ever sends one file
+        fileSize: MAX_PASTE_IMAGE_BYTES, // per file (default 50MB) — large phone photos / screenshots
+        files: 1, // paste-image sends one file per request (clients batch up to 20 requests)
         fields: 4, // small headroom for accompanying form fields
       },
     });