feat(theme): add 七十二候 micro-seasons theme + fix extension load resilience
Add Japanese 72 micro-seasons (Shichijūnikō) theme that changes colors
every ~5 days using traditional Japanese colors (伝統色). Preload sets
data-microseason attribute based on current date; CSS uses color-mix()
for automatic dark mode derivation.
Fix: use Promise.allSettled for parallel extension loading so one failed
extension doesn't block ext:all-loaded from firing, which prevented all
command registration (including theme switching).