fix: Agent Folder state not cleared when creating a new project

[dev-miro26]

Related Issue

Closes #1481

Description

When a user finishes a task and clicks the "+" button to create a new project, the Agent Folder panel on the right side still displays the previous task's file (e.g., file name header still shows, clicking it opens the old file) even though the "Files" section appears empty.

Root cause: The Folder component maintains local React state (selectedFile, fileTree, fileGroups, expandedFolders) that was never cleared when activeTaskId changed. The new project correctly sets selectedFile: null in the chatStore, but the component's local state persisted the previous task's values.

Fix:

1. Expanded the existing activeTaskId change useEffect to reset all local state (selectedFile, fileTree, fileGroups, expandedFolders), not just the hasFetchedRemote ref.
2. Added an else clause in the selectedFile sync useEffect to clear the local selectedFile when the new task has no selected file in the store.

Testing Evidence (REQUIRED)

eigent-issue1481.mp4

• I have included human-verified testing evidence in this PR.
• This PR includes frontend/UI changes, and I attached screenshot(s) or screen recording(s).
• No frontend/UI changes in this PR.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Contribution Guidelines Acknowledgement

• I have read and agree to the Eigent Contribution Guideline

[dev-miro26]

Hi @4pmtong
Could you please review this PR?

[4pmtong]

Thanks @dev-miro26 for contribution!

[dev-miro26]

Thanks @dev-miro26 for contribution!

Thank you!
Could you please merge this PR?

[dev-miro26]

Hi, @4pmtong
Could you merge my first PR?