spec: add SPEC-015 Configurable Notification Overlay

Comprehensive specification for on-screen notification when technician connects.

- Semi-transparent topmost window with configurable message, position, duration
- Dashboard admin settings page (enable/disable, message template, position, duration)
- Template variables: {{technician_name}}, {{company}}, {{time}}
- Agent displays overlay on StartStream, auto-hides after duration or manual dismiss
- Database: notification_config singleton table
- Protobuf: NotificationConfig message in StartStream
- Priority: P2, Effort: Medium (3-4 weeks)
- Added to roadmap under Core Remote Control

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docs/FEATURE_ROADMAP.md

@@ -63,6 +63,7 @@ Bringing GC to parity with GuruRMM's release engineering. Full plan: [SPEC-001](
 - [~] React/TS web viewer (`dashboard/src/components/RemoteViewer.tsx`) — embeddable session viewer
 - [ ] **Headless Linux mode (direct TTY access)** — P2 — Terminal-based remote access for Linux servers without GUI. PTY spawn (`openpty`), xterm.js web viewer, full ANSI/VT100 support. Enables server management, container debugging, emergency recovery via GuruConnect dashboard with audit logging. SSH replacement with centralized auth. ([SPEC-012](specs/SPEC-012-headless-linux-tty.md))
 - [ ] **Windows session selection and backstage mode** — P2 — Enumerate and switch between Windows user sessions (Terminal Services/RDP/Fast User Switching) and access Session 0 (backstage) for system-level admin tasks. ScreenConnect parity: session selector shows all logged-on users, instant switching without reconnect. Backstage mode provides terminal/command interface for services management without disrupting any user desktop. Critical for multi-user server environments. ([SPEC-013](specs/SPEC-013-session-selection-and-backstage.md))
+- [ ] **Configurable notification overlay on viewer connection** — P2 — Display a semi-transparent on-screen notification when a technician connects, showing technician name and company. Dashboard-configurable message template (supports `{{technician_name}}`, `{{company}}`, `{{time}}`), duration (5-60s), position (top-left/right, bottom-left/right, center), and dismissible behavior. Increases transparency and user awareness during remote support sessions. Compliance-friendly for privacy policies requiring user notification. ([SPEC-015](specs/SPEC-015-notification-overlay.md))
 - [ ] Multi-monitor switching — P2
 - [ ] File transfer — P3 (out of scope for native-remote-control v1)
 - [ ] Session recording — P3 (out of scope for native-remote-control v1)