refactor: replace all form actions by API endpoints

[nsarrazin]

We need to get rid of form actions, replace them with API endpoints with appropriate optimistic updates/invalidations.

The following actions need to be replaced:

• src/routes/conversation/[id]/+page.server.ts #1701
• src/routes/conversations/+page.server.ts #1702
• src/routes/login/+page.server.ts #1704
• src/routes/logout/+page.server.ts #1705
• src/routes/settings/(nav)/assistants/[assistantId]/+page.server.ts (delete, report, subscribe, unsubscribe, deny, approve, request, unrequest)
• src/routes/settings/(nav)/assistants/[assistantId]/edit/+page.server.ts
• src/routes/settings/(nav)/assistants/new/+page.server.ts
• src/routes/tools/[toolId]/+page.server.ts (delete, report, deny, approve, request, unrequest)
• src/routes/tools/[toolId]/edit/+page.server.ts
• src/routes/tools/new/+page.server.ts