[Feature]: Support Custom Notification Channels (Webhook / Bark / ntfy / Telegram)

### Problem or use case

When running long tasks or using Orca remotely, desktop notifications are often not enough.

I would like to receive notifications on my phone or through other services when:

- An agent completes a task
- An agent requires user input
- An error occurs
- A task fails

Supporting custom notification channels (such as Bark, ntfy, Telegram, Discord, Slack, or a generic webhook) would make it much easier to stay informed without keeping the Orca window open.

### Proposed solution

## Problem

Currently Orca only supports built-in notifications. When running long tasks or using Orca remotely, it would be useful to receive notifications through third-party services.

Examples include:

- Bark (iOS)
- ntfy
- Telegram Bot
- Discord Webhook
- Slack Webhook
- Generic HTTP Webhook

## Proposed Solution

Add support for configurable notification channels.

For example, in **Settings → Notifications**, users could choose one or more notification channels:

- Desktop Notification
- Sound
- HTTP Webhook
- Bark
- ntfy
- Telegram
- Discord
- Slack

Even better, implement a **generic HTTP Webhook** that allows users to configure:

- Request URL
- HTTP Method (GET/POST)
- Custom Headers
- Request Body / JSON Template

This would make it possible to integrate Orca with many notification services without adding dedicated support for each platform.

## Example Use Cases

- Send a Bark notification when an agent finishes.
- Notify my phone when Orca requires attention.
- Send task completion messages to Discord or Slack.
- Trigger Home Assistant automations.

### Alternatives or additional context

A generic webhook implementation would cover most notification services, including Bark, ntfy, Discord, Slack, Home Assistant, and many self-hosted solutions.

This approach would also reduce maintenance effort compared with implementing each notification provider individually.