refactor: replace all server-side load functions by universal load functions

[nsarrazin]

We need to replace all the +page.server.ts and +layout.server.ts by +page.ts and +layout.ts. In order to do this, we need to create more routes under src/routes/api with +server.ts GET endpoints, which we can then fetch inside the universal load functions.

The following server-side load functions need to be replaced:

• src/routes/+layout.server.ts
• src/routes/assistant/[assistantId]/+page.server.ts
• src/routes/assistants/+page.server.ts
• src/routes/conversation/[id]/+page.server.ts
• src/routes/login/callback/+page.server.ts
• src/routes/models/[...model]/+page.server.ts
• src/routes/settings/+layout.server.ts
• src/routes/tools/+page.server.ts
• src/routes/tools/[toolId]/+layout.server.ts