Add keyboard shortcuts for common board actions #256

Problem

Power users have no keyboard access beyond the global search (Cmd/Ctrl+K). Creating and editing tasks requires mouse/touch for everything.

Definition

A minimal, discoverable keyboard layer for the board, active only when the user is not typing:

• n: new task (opens the create modal with the current board/project context)
• ?: cheat sheet overlay listing all shortcuts
• Arrow keys: move focus across cards (roving tabindex, respecting current filters and column order)
• Enter: open the focused task; e: open it on the Details tab; Esc: close modal/overlay
• Cmd/Ctrl+K stays as is (global search)

Scope

In scope:

• Board page only (project board and general board).
• Roving focus: exactly one card is focusable at a time; focus ring visible.
• The overlay shows the actual shortcuts per platform (Cmd vs Ctrl, mobile hint: not available).
• All shortcuts ignored while focus is in an input, textarea, contenteditable or any modal (except Esc).

Out of scope:

• Reordering cards by keyboard (drag stays mouse/touch).
• Shortcuts in other pages (Today, Summary, Routines) beyond Esc.
• Custom/remappable keys.

Technical notes

• Cards already expose data-task and role=button; add a roving tabindex managed by a small hook in BoardPage.
• NewTaskModal is already opened via modal-context openNewTask(defaults), reuse it.
• The ? overlay: new component, dismissible with Esc or ?, no route change.
• Mind existing listeners: SearchModal (Cmd/Ctrl+K) captures at window level with input guards; follow the same pattern (input/textarea/contenteditable guard before handling single-key shortcuts).
• i18n es/en required; shortcuts themselves stay non-localized.

Acceptance criteria

• n opens the create modal with the current project prefilled.
• Arrow keys cycle cards in visual order and skip filtered-out/archived cards.
• ? opens the overlay; Esc closes it; no shortcut fires while typing in any field.
• Existing E2E/Playwright board flows keep passing.