ci: build macOS releases on macos-26 runners, realign traffic-light insets

[Tryanks]

Problem

Release builds installed on macOS 26 show the legacy small traffic-light window buttons instead of the new larger ones.

Root cause: macOS 26 renders the new (Liquid Glass) traffic lights only for apps whose Mach-O LC_BUILD_VERSION records the macOS 26 SDK. The release workflow built on macos-14 runners, which ship at most Xcode 16 (macOS 15 SDK), so released binaries record sdk 15.x and AppKit renders them in compatibility mode with the old small buttons. Verified locally: the same source built with Xcode 26.5 records sdk 26.5 and gets the new buttons.

Changes

CI (release.yml, ci.yml) — macOS jobs move from macos-14 to macos-26:

• macos-26 is GA since 2026-02-26 and ships Xcode 26, so release binaries link the macOS 26 SDK and get the new chrome automatically.
• The x64 build keeps cross-compiling from the arm64 runner (--target x86_64-apple-darwin), same as before; the Xcode 26 SDK still supports x86_64.
• Also moves off an image that is deprecated as of 2026-07-06 and retires 2026-11-02 (brownouts start October).

UI insets — measured the new traffic-light geometry on macOS 26.5 (14pt buttons, 60pt-wide group ending at x=72pt with the existing placement) and adjusted the four constants tuned for the old 12pt buttons:

• crates/app/src/main.rs: traffic_light_position y 18 → 19, so the 14pt buttons sit exactly centered in the 52px top strip.
• crates/ui/src/sidebar.rs / settings_page.rs: TRAFFIC_LIGHT_INSET 74 → 80, keeping ~8pt clearance for the branding next to the wider group.
• crates/ui/src/chat.rs: collapsed-sidebar header inset 36 → 40 for the same reason.
• Non-macOS inset values unchanged.

Verification

• cargo build --release --locked (LTO thin + strip, same as CI): otool -l shows LC_BUILD_VERSION sdk 26.5.
• Dry-ran the release.yml macOS packaging steps locally (Info.plist lint, ditto zip, hdiutil DMG) — all pass.
• Visual check with before/after screenshots of the expanded sidebar, collapsed sidebar, and settings page: branding/title clear the new light group by ~8pt and the group is vertically centered in the top strip.
• cargo fmt --all --check and cargo check pass.

Notes

• macos-latest intentionally not used: the label only finished migrating to macOS 26 around 2026-07-15 and will drift again; pinning macos-26 keeps the linked SDK deterministic.
• On pre-26 macOS the 1pt position nudge and 6pt wider inset are visually harmless (slightly more breathing room next to the old smaller buttons).

Co-Authored-By: kimi