Use Remix Route Conventions docs to make a "return" header over the form

[thadk]

First steps:

• Copy /single ( that is, _heat+/single.tsx ) into a new route inside _heat+ folder, that can be accessed with /app/ aka https://heat-stack.fly.dev/app/.
  • That will include https://heat-stack.fly.dev/app/single but any other file in that same folder too.
  • The header should not be on the home page https://heat-stack.fly.dev/.
• Make the dark blue "return" header bar appear over single.tsx and any other files in that /app/ route.
• ShadCN Chevron left icon
• style the header and add a Return to Site link that goes to the root page on the domain ( / )

Docs: https://remix.run/docs/en/main/discussion/routes

The notes in PR may be helpful beyond the documentation: #125

Advanced:

• Build the sidebar within the header, all wrapped around single.
• Use the same grid ratios as in the figma design.
• export image from figma
• No need to implement "start over", "dev upload case" or "report issue" functionality, please just make the buttons/links.