#21585 feat(cmux-next): add priority sidebar view

What changed

The existing SidebarPriority projection is now rendered as a real sidebar view. The sidebar has a visible Priority entry point, complete localized title/empty-state/status copy, accessible priority rows, keyboard navigation and activation, and selection callbacks that reveal the selected workspace. Pending workspace rows are excluded, and toggling routes preserves each route's scroll position.

Design reference match

This follows the quiet, scannable sidebar treatment in the Codex grouped-priority/chat reference and Claude's overflow navigation reference: semantic status remains compact, the entry point stays in the existing sidebar control rhythm, and the Priority route avoids new motion or boxed dashboard tiles.

Validation

• Exact PR head: d18599c139da7b23e1ecc99f0b92a2df18f22900
• Rebased onto live feat-cmux-next base c6f31b26494fb7a07f5349590bd077b96b17a793 before publication.
• Focused swiftc -parse passed for all changed Swift files.
• Focused catalog audit passed: 4 new keys × 21 locales, all translated.
• git diff --check: clean.
• Hosted workflow cmux-next run 39129096783: success; exact-head PR wait reports SUCCESS, requiredMissing [], and CLEAN.
• Native macOS build is delegated to managed fleet checks; it was not run on this M5 laptop.
• This is native AppKit sidebar code, so there is no HTTP live URL; managed fleet/dogfood verification is the runtime step.

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