fix: prevent blank screen when navigating to non-existent channels

[raaymax]

Summary

• Gate Router render on isLoading to prevent components rendering before channel validation completes
• Upsert fetched channel into MobX store during route loading so it's available for rendering
• Change getThread() and getMessages() to return null instead of throwing when channel not found (consistent with getPins/getSearch)
• Add null guards in SideConversation, MainConversation (Desktop + Mobile), Conversation, and MessageInfo components
• Add null check in channels.find() for non-existent channel API responses

Test plan

• Navigate to a non-existent channel ID (e.g. /#/000000000000000000000000) — should show 404 error page, not blank screen
• Navigate between valid channels — should load without crash
• Open a thread side panel — should render correctly
• TypeScript check passes (cd app && npm run types) — no new errors introduced