Make Quick Open, outline, and library navigation fully keyboard-operable

[clintecker]

Problem

Several major navigation surfaces are visually present but not yet complete for keyboard-first use.

Why it matters for target users

Keyboard-first writers, engineers, researchers, and accessibility users need to move through documents, headings, tabs, and search results without falling back to pointer-only interactions.

Evidence / code or doc references

• App/macOS/Sources/LibrarySidebar.swift tracks a highlighted Quick Open result and opens it on submit, but there is no visible arrow-key handling to move the highlight.
• App/macOS/Sources/ReaderScreen.swift renders outline rows as flat buttons; the handoff calls for collapsible chevrons in a ruled tree.
• App/macOS/Sources/LibrarySidebar.swift exposes rename/reveal mainly through context menus.
• docs/design/handoff.md specifies Quick Open, outline collapse, tab switching, and keyboard-first shortcuts as core interactions.
• Apple macOS guidance treats keyboard shortcuts and accessible interaction alternatives as important for both power use and accessibility.

Suggested direction

• Add Up/Down/Home/End handling for Quick Open and library search results, including visible selection and wrap behavior.
• Add outline collapse/expand state with keyboard commands and accessible disclosure semantics.
• Ensure library rows support keyboard open, rename, reveal, and move/delete/duplicate commands as those workflows land.
• Keep shortcut ownership in menus once issue Move hidden keyboard shortcuts into real macOS menu commands and toolbar affordances #12 promotes hidden shortcuts into real commands.

Acceptance or test ideas

• A user can launch Quoin, open Quick Open, choose a result, navigate the outline, rename a document, and switch tabs using only the keyboard.
• UI tests exercise Quick Open arrow selection and outline collapse/expand.
• VoiceOver focus order follows sidebar -> tabs/editor -> outline -> status/find/export controls without dead ends.