Switch bar sections with Command-Left and Command-Right

[alvst]

⌘← and ⌘→ move between Clipboard and the Pinboard tabs without reaching for the mouse. Plain arrow keys keep moving the card selection, so the two navigations don't fight.

What changed

• ⌘← / ⌘→ cycle Clipboard and each Pinboard in the order the tab bar already presents them.
• Card navigation is untouched: unmodified arrows still move the selection within the current section.

Screenshots

No visual change — this is a keyboard-only addition. It adds no chrome; ⌘←/⌘→ perform the same section switch a click on the tab row already does, so the resulting UI is identical to what a click produces today.

For context, a Pinboard reached by the new shortcut — identical to reaching it by clicking the tab:

Notes for review

• No dependencies; merges in any order.
• Checked against upstream's existing bindings: the new cases are ⌘-guarded and sit ahead of the printable-character search fall-through, so no existing shortcut changes meaning.
• Out of scope: no wrap-around preference and no menu items for the same action.

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This feature should be bundled into v2.

Part of #80.