Leftover todo from svelte 5 migration

[nsarrazin]

Migrated the app to Svelte 5 syntax in #1685, seems to have worked well.

I started updating some of the syntax but we still have some leftover use of svelte/legacy in some files:

• src/lib/components/MobileNav.svelte #1697
• src/lib/components/ScrollToBottomBtn.svelte
• src/lib/components/chat/ChatMessage.svelte
• src/lib/components/chat/ChatWindow.svelte
• src/lib/components/chat/FileDropzone.svelte
• src/routes/+layout.svelte
• src/routes/conversation/[id]/+page.svelte
• src/routes/tools/+page.svelte
• src/routes/tools/ToolInputComponent.svelte

Getting rid of it is not super high priority but it would make us more future proof.

We also make use of createEventDispatcher and the $page store which have both been deprecated. Updating these will require a bit more work throughout the app and should be done in small sections if possible so we can thoroughly test the changes.