✨ feat: introduce unified sidebar redesign

[vaayne]

Summary

• replace the three-mode sidebar UI with a single unified sidebar surface
• add compact top-level summary and "Now" sections for active agent work
• simplify worktree rows and selected detail rows to make scanning easier

Changes

• remove the visible Workspaces/Tasks/Agents mode switch from the sidebar container
• redesign WorktreeSidebarView into the primary unified navigation surface
• simplify WorktreeRowView to show one primary status badge with quieter metadata
• deduplicate project action menus and tighten selected detail window rendering
• add required sidebar localization strings in English and Simplified Chinese

Verification

• swift build

Notes

• sidebar-unified-mockup.html remains local and is not included in this PR

[chatgpt-codex-connector]

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[0xbentang]

The unified side bar is much better than before, thank you!