keyboard: board card navigation (j/k/arrows + actions)

Focused-card selection on the board: j/k or arrows move it, Enter opens,
e archives, # pins, x trashes the focused card (actions gated off the Trash
view). The selection is a brand ring on the card; it scrolls into view and
clamps as the list/view changes. A window keydown listener on BoardView,
ignored while typing, on a button/link, or while the editor modal is open.
Added the card keys to the ? cheat-sheet.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/components/AppShell.vue

@@ -30,6 +30,9 @@ const shortcuts = [
   { label: "Go to Board", keys: ["g", "b"] },
   { label: "Go to Graph", keys: ["g", "g"] },
   { label: "Go to Reminders", keys: ["g", "r"] },
+  { label: "Move card focus", keys: ["j", "k"] },
+  { label: "Open focused card", keys: ["Enter"] },
+  { label: "Pin / archive / trash card", keys: ["#", "e", "x"] },
   { label: "Save & close (editor)", keys: ["⌘/Ctrl", "Enter"] },
   { label: "Save & new (composer)", keys: ["Shift", "Enter"] },
   { label: "Dismiss / close", keys: ["Esc"] },