feat: allow templating in the Signal notificaiton provider

[karthikeya-333]

In this pull request, the following changes are made:

Added custom templating option for Signal notififcation provider.

Backend (server/notification-providers/signal.js):

• Updated the send() function to correctly handle message text when the custom template option is enabled or disabled for the Signal provider.

Frontend (src/components/notifications/Signal.vue)

• Added UI support to allow users to toggle the custom message template feature.
• This aligns Signal with other notification providers where the same feature already exists.

Resolves #6889

Please follow this checklist to avoid unnecessary back and forth (click to expand)

• ⚠️ If there are Breaking change (a fix or feature that alters existing functionality in a way that could cause issues) I have called them out
• 🧠 I have disclosed any use of LLMs/AI in this contribution and reviewed all generated content.
  I understand that I am responsible for and able to explain every line of code I submit.
• 🔍 Any UI changes adhere to visual style of this project.
• 🛠️ I have self-reviewed and self-tested my code to ensure it works as expected.
• 📝 I have commented my code, especially in hard-to-understand areas (e.g., using JSDoc for methods).
• 🤖 I added or updated automated tests where appropriate.
• 📄 Documentation updates are included (if applicable).
• 🧰 Dependency updates are listed and explained.
• ⚠️ CI passes and is green.

Screenshots for Visual Changes

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[github-actions]

Hello and thanks for lending a paw to Uptime Kuma! 🐻👋
As this is your first contribution, please be sure to check out our Pull Request guidelines.
In particular: - Mark your PR as Draft while you’re still making changes - Mark it as Ready for review once it’s fully ready
If you have any design or process questions, feel free to ask them right here in this pull request - unclear documentation is a bug too.

[CommanderStorm]

Works for me, thanks

[CommanderStorm]

Seems ready, please ping me when you think so too (it is currently a draft).

[karthikeya-333]

Seems ready, please ping me when you think so too (it is currently a draft).

Done, changed it to ready state.

[github-actions]

@karthikeya-333 congrats on your first contribution to Uptime Kuma! 🐻
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