fix&style: conversation can't create when join existing group after started, improve layout

[howard9199]

This pull request includes several changes to improve the layout and functionality of the chatroom and session components, as well as updates to the server-side logic for creating conversations and user sign-in. The most important changes include modifications to the Chatroom.svelte, HostView.svelte, and ParticipantView.svelte components, as well as updates to the Firebase server and sign-in API.

Improvements to chatroom layout:

• src/lib/components/Chatroom.svelte: Added minimum height to the chat container and adjusted the layout of the message input area to improve responsiveness and usability. [1] [2] [3]

Enhancements to session views:

• src/lib/components/session/HostView.svelte: Centered the session code display for better visual alignment.
• src/lib/components/session/ParticipantView.svelte: Added a minimum height to the participant view container to maintain consistent layout.

Updates to server-side logic:

• src/lib/server/firebase.ts: Modified the createConversation function to check for existing conversations by user ID, ensuring that duplicate conversations are not created.
• src/routes/api/auth/signin/+server.ts: Updated the default values for title and bio fields during user sign-in to provide initial data.

Copilot reviewed 2 out of 5 changed files in this pull request and generated no comments.

Files not reviewed (3)

• src/lib/components/Chatroom.svelte: Language not supported
• src/lib/components/session/HostView.svelte: Language not supported
• src/lib/components/session/ParticipantView.svelte: Language not supported

Comments suppressed due to low confidence (1)

src/lib/server/firebase.ts:47

• The comment should be translated to English for consistency. Suggested translation: 'Check if a conversation already exists for the user'.

// 檢查是否已存在該使用者的對話