chore: add default app icon

[srtaalej]

Summary

• Adds a default app icon at assets/icon.png
• The Slack CLI auto-detects this path and uploads it when running locally

Test plan

• Verify the icon renders correctly in the PR diff
• Run slack run --experiment set-icon and confirm the icon appears on the app

[WilliamBergamin]

Thanks for working on this, I'm not sure if this change should be applied to this project, the project is not setup to work with the CLI adding the icon will have no effect 😅

[zimeg]

@WilliamBergamin I'm curious to open an adjacent PR for default CLI support? We have a solid create experience that installs packages needed but that adds background time to CI setups using this template 🧪

[WilliamBergamin]

@zimeg yess we can introduce support for the CLI in this project 💯 Even if it add background time to CI I think its fine, if the time becomes a problem in the future then we will address it

[zimeg]

LGTM - I look forward to set-icon experiments concluding stable! 🔮

[srtaalej]

@WilliamBergamin thanks for letting me know this doesnt have CLI support! i did get an approval from @zimeg though - does that mean i can merge now or we wait for CLI support 🔮 🤔