fix(launchpad): cancel stale flash timer

[FerroxLabs]

Closing or navigating away from the Launchpad picker during its 600 ms selection flash left a component-owned timeout alive. The delayed state update could run after unmount; the full Linux suite exposed it as an unhandled window is not defined exception.

This change retains one flash timeout, cancels it when a newer pick replaces it, clears it on expiry, and disposes it on unmount. Focused regressions cover flash duration, replacement, unmount/remount isolation, inert pinned/capped cards, and exact Escape-listener cleanup.

Validation:

• ASTRA bounded lifecycle review completed within the two-round limit.
• Required prek passed.
• Focused LaunchpadPicker suite: 15 passed.
• Exact combined Linux aggregate: 21,524 Vitest passed / 53 skipped; 285 Bun-native passed / 0 failed; no unhandled callback error.