build: simplify Makefile targets and add live-reload preview

Rename site_* targets to bare names (install, fetch_stats, build,
preview). Replace the static preview target with a watchmedo-driven
live-reload loop so file changes trigger automatic rebuilds. Make
the output directory creation idempotent (exist_ok=True) and static
copy incremental (dirs_exist_ok=True) so repeated builds don't wipe
output on each run.

Co-Authored-By: Claude <[email protected]>