fix: sidebar spacing grid and window corner radius

[arzafran]

What this does

Cleans up the sidebar layout so everything lines up the way Slack's sidebar does: one left gutter, titles that all start at the same x, status on the right, and the footer buttons on the same icon column as the rows. Also fixes the terminal card corner so it follows the window's corner instead of looking tighter.

Summary

• Rows sit 8pt from the sidebar edge (the extra 6pt per-row inset is gone). Leading icons (pin, worktree badge) live in a fixed 16pt column centered 24pt from the edge; titles start at 38pt whether or not an icon is present.
• Agent status (Working / Needs input / Idle) and the unread badge render trailing, after the title, so dynamic text no longer shifts titles.
• The CLI's Idle status entry is filtered like the other agent-state duplicates, so a row no longer shows both Working and Idle.
• Footer help and usage buttons are 24pt boxes with 16pt glyph slots on the row icon column; the update pill follows at the same gap. The 44pt offset-based layout and its helpers are removed.
• Header controls' trailing inset is derived from that column, so the + centers 24pt from the edge, mirroring the row close button. The shortcut-hint x offset no longer moves the icons.
• windowCornerRadiusDefault goes from 12 to 16 (measured against the macOS 26 window corner), so the content card (12) and sidebar panel (10) stay concentric with the window.

Test plan

• CI unit tests green (testFooterGlyphsShareTheSidebarRowIconColumn replaces the old pitch test)
• Debug build: pin icon, help icon, and usage icon centers all sit 24pt from the sidebar edge; titles align with and without a pin
• Debug build: bottom-right terminal corner gap is even along the curve