Add fixed chat textarea option to AccessibilitySettings (#809)

# PR Checklist
- [ ] Have you checked if it works normally in all models? *Ignore this
if it doesn't use models.*
- [ ] Have you checked if it works normally in all web, local, and node
hosted versions? If it doesn't, have you blocked it in those versions?
- [ ] Have you added type definitions?

# Description
This pull request introduces the following changes:
- Adds a new "fixedChatTextarea" option to control the positioning of
the chat window text area.
- Modifies Shift+Enter functionality to align with updated input
handling requirements.

To be honest, I'm not entirely sure if this approach is correct. Since
I've developed with React and Next.js, I'm still a beginner when it
comes to Vite. But as a PC user, I think this method will be convenient
to use.

src/ts/storage/database.svelte.ts

@@ -694,6 +694,7 @@ export interface Database{
     }
     globalscript: customscript[],
     sendWithEnter:boolean
+    fixedChatTextarea:boolean
     clickToEdit: boolean
     koboldURL:string
     advancedBotSettings:boolean