feat: thread navigation

[krantheman]

Using next/previous buttons

Closes: #273

Screen.Recording.2025-09-18.at.12.11.47.PM.mov

Using arrow keys

Closes: #272

Screen.Recording.2025-09-18.at.12.13.00.PM.mov

[Copilot]

Pull Request Overview

This PR implements thread navigation functionality allowing users to navigate between email threads using both UI buttons and keyboard shortcuts. The feature adds previous/next navigation buttons in the thread view and enables arrow key navigation when the mail list is focused.

• Added previous/next thread navigation buttons in the MailThread component
• Implemented arrow key navigation (up/down) for thread switching in split view
• Enhanced visual feedback with improved highlighting and click detection

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
frontend/src/pages/MailboxView.vue	Implements keyboard navigation logic, thread offset calculations, and passes thread data to MailThread component
frontend/src/components/MailThread.vue	Adds previous/next navigation buttons with proper disabled states and emits navigation events
frontend/src/components/MailListItem.vue	Adds border styling to avatar component

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}