feat(tui): add config options for TUI appearance customization

[eirikb]

Summary

Hi there.
This PR adds three new TUI config options for customizing the interface:

• tui.hide_startup_tips - Hide the startup command tips (defaults to false)
• tui.hide_session_header - Hide the session header with version/model info (defaults to false)
• tui.input_placeholder - Set a custom input placeholder instead of random examples (optional)

Making it go from this:

to this:

Changes

• Added config fields to the Tui struct and wired through Config
• Updated chat widget, footer, and history cell rendering to respect these settings
• Added documentation to docs/config.md
• Added tests for config parsing

Testing

• All config tests pass (75 tests)
• cargo build --all-targets succeeds
• cargo fmt -- --config imports_granularity=Item
• cargo clippy --tests
• Manually tested each option in the TUI

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[eirikb]

I have read the CLA Document and I hereby sign the CLA

[etraut-openai]

Thanks for the contribution. We've updated our contribution guidelines to clarify that we're currently accepting contributions for bugs and security fixes, but we're not generally accepting new features at this time. We need to make sure that all new features compose well with both existing and upcoming features and fit into our roadmap. If you would like to propose a new feature, please file or upvote an enhancement request in the issue tracker. We will generally prioritize new features based on community feedback.