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/config/buffer-limits.ts

@@ -96,3 +96,18 @@ export const TRIM_RESPAWN_BUFFER_TO = 512 * 1024; // 512KB
  * which is enough to extract metadata from the first few JSONL lines.
  */
 export const FILE_PEEK_BYTES = 8 * 1024 - 1; // 8KB (inclusive end offset)
+
+// ============================================================================
+// Paste-Image Upload Limits
+// ============================================================================
+
+/**
+ * Maximum size (bytes) of a single image uploaded via POST
+ * /api/sessions/:id/paste-image. The mobile picker / drag-drop / paste paths
+ * send one file per request (the client uploads up to MAX_PASTE_IMAGES of them
+ * per batch), so this caps each individual file, not the batch. Generous enough
+ * for full-resolution phone photos and large screenshots; the client downscales
+ * very large images before upload, so legitimate uploads land well under this.
+ * Override: CODEMAN_MAX_PASTE_IMAGE_BYTES (bytes)
+ */
+export const MAX_PASTE_IMAGE_BYTES = parseInt(process.env.CODEMAN_MAX_PASTE_IMAGE_BYTES || '') || 50 * 1024 * 1024; // 50MB