Keyboard door to the thread and file tree context menus #590

User story

As a person who uses eve from the keyboard, I want to open a thread's or a file's actions menu without a mouse, so that I can rename, move or delete it from the keyboard.

Acceptance criteria

• A key on a focused thread row opens that thread's actions menu.
• A key on a focused file tree item opens that item's actions menu.
• The arrow keys move between the menu items, and Return runs the focused item.
• Escape closes the menu and returns focus to the row that opened it.

What I'd notice

Keyboard users can rename, move and delete threads and files; nothing changes on screen for mouse users.

Context

Found while working #326. Today only right-click and long-press open these menus (Rename, Move to folder, New File, Delete). #326 makes the rows and their buttons reachable but adds no key for the context menu.

Tasks

• Planner contract: the key (Shift+F10 / context-menu key) and focus rules
• Open the thread and file tree menus from the keyboard
• Specs that drive each criterion from the keyboard only