Add localhost screenshot endpoint and email-upload guide

- Electron main now exposes http://127.0.0.1:9990/screenshot that returns
  a PNG via CDP Page.captureScreenshot (GNOME Wayland blocks external
  capture, and webContents.capturePage() hangs on this compositor). The
  endpoint is localhost-only.
- EMAIL_UPLOAD.md documents the n8n IMAP -> Samba share workflow for
  Skylight-style email-to-photo-frame uploads.

EMAIL_UPLOAD.md

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+# Email-to-Photo-Frame Upload
+
+Photos emailed to a dedicated mailbox are automatically saved to the
+photo-frame Samba share, where the kiosk picks them up on its next scan
+(PhotoManager rescans every 10 minutes).
+
+## Three moving parts
+
+### 1. Mailbox to receive photos
+
+Easiest path: a dedicated Gmail account (e.g. `deathstarhomephotos@gmail.com`).
+
+- Enable 2FA on the Google account
+- Generate an **app password** under Security → App passwords (needed for IMAP,
+  since n8n's IMAP node uses username/password auth)
+- Share the address with family members
+
+Lock it down in the n8n workflow by whitelisting sender addresses; reject
+anything else so strangers can't spam the frame.
+
+### 2. Samba share visible to n8n
+
+n8n runs on 192.168.1.254 and needs to write to the same Samba share the
+kiosk reads from. Two options:
+
+**Option A — Mount the share inside the n8n container**
+
+```bash
+# On the n8n LXC host (192.168.1.254), install cifs-utils and mount:
+apt install -y cifs-utils
+mkdir -p /mnt/family-photos
+echo '//SERVER/SHARE /mnt/family-photos cifs credentials=/etc/samba/credentials-n8n,uid=1000,gid=1000,rw,_netdev,x-systemd.automount 0 0' >> /etc/fstab
+# credentials file with chmod 600
+mount -a
+```
+
+Then bind-mount `/mnt/family-photos` into the n8n Docker container so the
+Write Binary File node can target it at a path like `/data/family-photos/`.
+
+**Option B — SSH to the fileshare LXC (192.168.1.193)**
+
+Give n8n an SSH key to the fileshare host. Use the Execute Command node to
+`scp` / `cat > file` onto the share. Simpler to configure than cifs, but
+adds SSH surface area on the fileshare box.
+
+### 3. n8n workflow
+
+Pseudocode:
+
+```
+IMAP Email Trigger (mark read, download attachments, poll every 2 min)
+  → IF From ∈ [your-email, wife-email]
+  → Split attachments
+  → IF mimeType startsWith "image/"
+  → Rename: ${yyyyMMdd_HHmmss}_${randomHex}.${ext}
+  → Write Binary File → /data/family-photos/${filename}
+```
+
+Nodes used:
+- **Email Trigger (IMAP)** — reads Gmail (server `imap.gmail.com`, port 993, SSL)
+- **IF** — sender whitelist guard
+- **Item Lists / Split In Batches** — one item per attachment
+- **IF** — filter on `$binary.attachment_0.mimeType`
+- **Set** — build filename
+- **Write Binary File** — write to the mount path
+
+Kiosk picks them up on the next rescan (up to 10 min), or restart the app
+to force an immediate scan.
+
+## Hardening
+
+- Limit attachment size (e.g. reject > 30 MB per file) to avoid filling
+  the share
+- Strip EXIF if you don't want location data persisted on the share
+- Keep an audit trail: add an extra branch that appends sender + filename
+  to a log file
+
+## Credentials I'll need to build the workflow
+
+- Gmail address + app password (or OAuth refresh token)
+- Which option above for the Samba mount (A or B)
+- If option B, SSH key + fileshare path to write to
+- List of allowed sender emails