v1.6.6 release

Quality-of-life on the X button, the apply modal, and Backups. The X minimizes to the system tray for new installs while your saved choice still wins. The Apply Progress wall is gone: close the modal mid-apply, navigate to Catalog or About, then pick it back up from the activity dock. And the date-grouped Backups view divides itself into month headers, so months of history read at a glance instead of a flat scroll.

See CHANGELOG.md [1.6.6] for the full Added / Changed / Fixed lists.

Gates

• cargo fmt --all -- --check: exit 0
• cargo clippy -p dlssync --lib --tests -- -D warnings: exit 0
• cargo test --workspace --lib: 367 tests, 12 crates, 0 failed
• svelte-check: 0 errors / 0 warnings
• vitest: 51 files, 452 tests, 0 failed (includes the new a11y.test.ts axe-core sweep + bundleSizeBudget.test.ts)
• pnpm build: 4.15s, JS 230.87 KB gzip / CSS 66.81 KB gzip (within 250 / 75 KB budgets)
• pnpm tauri build --debug --no-bundle: 45.5s, target/debug/dlssync.exe boots against real user data (catalog + backups DB + settings load clean)

Major changes

• T-2 apply pipeline refactor: pure decision helpers extracted to commands/apply_decisions.rs with a focused 14-test cargo suite. apply.rs hot path trims 325 lines, stays the orchestrator.
• T-9 <FlyoutShell>: shared backdrop / dialog frame / Escape-close / fly transition primitive consumed by Driver History + Catalog Versions. VersionPickerPopover stays separate (structurally divergent).
• T-11 axe-core a11y contract: axe-core@^4.10.3 added at workspace root. tests/contracts/a11y.test.ts renders Checkbox / CounterPill / BrandMark / Toast and gates on zero critical/serious WCAG violations.
• T-6 Backups month headers: month dividers inline in the existing {#each filtered} loop when groupBy === "date". No parallel view, no god-component plumbing.
• Round 0 backend reliability: coordinate_singleton helper + ensure_system_info coordination + fs::copy wrapped in spawn_blocking. close_to_tray default ON for new installs, explicit false preserved.

26 files changed, +1032 / -454 net.