Add TestBackend-based widget rendering tests with insta snapshots

[bug-ops]

Parent: #542

Problem

Only hyperlink.rs uses Buffer directly for testing. Widgets splash, input, confirm, help, skills, memory, resources, and status have no rendering tests.

Requirements

• Add dev-dependency: insta with redactions feature
• Create test helper: fn test_terminal(w: u16, h: u16) -> Terminal<TestBackend>
• Create test helper: fn render_widget<F>(w: u16, h: u16, f: F) -> String returning buffer as string
• Write snapshot tests for each untested widget: splash, input, confirm, help, skills, memory, resources, status
• Test each widget in multiple states (e.g., input Normal vs Insert mode, confirm with different prompts)
• Use insta::assert_snapshot! for all assertions

Acceptance criteria

• Every render function has at least 2 snapshot tests
• All snapshots are committed and reviewed