chore: Update PATTERNS.md: change screens.push + pop to be used via Context

[marceloadsj]

What

Complement the changes that fixed issue: #279

Updating PATTERNS.md doc to make use of the ui.push_screen() + ui.pop_screen() in the example where ui.screen() is being used as well.

Why

To make sure the sample code shown in the docs compile without errors.

Checklist

Required

• cargo fmt -- --check passes
• cargo check --all-features passes
• cargo clippy --all-features -- -D warnings passes
• cargo test --all-features passes
• cargo check --examples --all-features passes

Public API Changes

• New public items have doc comments (///)
• Widget follows the Response pattern
• State struct in widgets.rs, impl on Context, re-export in lib.rs
• Breaking change? Update CHANGELOG.md and note it here

If Adding a Widget

• Calls register_focusable() if interactive
• Consumes handled key events
• Uses self.theme.* for default colors
• Example added or existing example updated

If Applicable

• Tests added
• Animation uses build-time pre-sort (not per-frame)
• No unwrap() in Result-returning functions