Truncate noisy build-failure output, fix the recurring terrain death

The web UI was showing a raw ~500k-character stdout/stderr dump on a
failed build — go-pmtiles' extract logs one progress line per chunk,
which is fine on a terminal (it overwrites itself) but not captured
non-interactively. command_output now keeps just the tail, and the
status line renders it with preserved line breaks in a scrollable box
instead of one unreadable blob.

Root cause of the actual failure (terrain build dying ~11 minutes in
with an HTTP/2 stream reset): go-pmtiles' default --overfetch=0.05
merged this multi-country planet.pmtiles extract into a single ~60GB
request with no way to resume after a dropped stream. --overfetch=0
splits it into many smaller requests instead.

docs/development.adoc

@@ -71,13 +71,23 @@ and similar, varying by region):
 
 [source,shell]
 ----
-pmtiles extract https://download.mapterhorn.com/planet.pmtiles planet-extract.pmtiles --bbox=west,south,east,north --maxzoom=15
+pmtiles extract https://download.mapterhorn.com/planet.pmtiles planet-extract.pmtiles --bbox=west,south,east,north --maxzoom=15 --overfetch=0
 # only if a regional chunk covers the bbox too (see download_urls.json) —
 # most bboxes hit exactly one, a bbox straddling a chunk boundary hits two:
-pmtiles extract https://download.mapterhorn.com/<chunk>.pmtiles chunk-extract.pmtiles --bbox=west,south,east,north --maxzoom=15
+pmtiles extract https://download.mapterhorn.com/<chunk>.pmtiles chunk-extract.pmtiles --bbox=west,south,east,north --maxzoom=15 --overfetch=0
 pmtiles merge planet-extract.pmtiles chunk-extract.pmtiles data/terrain.pmtiles
 ----
 
+`--overfetch=0`: the default (0.05) merges nearby byte ranges into fewer,
+bigger requests to save round-trips — for `planet.pmtiles` (global z0-12,
+tens of GB) over a bbox spanning several countries this can collapse the
+whole extract into a *single* multi-GB request with no way to resume after
+a mid-transfer HTTP/2 stream reset. `--overfetch=0` trades that one
+fragile mega-request for many small ones instead (spread over
+`--download-threads`, default 4) — slower end to end, but a dropped
+connection only loses one small request's worth of progress, not the
+whole extract.
+
 Tiles are 512px WebP, not the 256px PNG this app's *online* Terrarium
 source uses — `public/js/dem.js`'s own point-elevation lookups
 (`shadow.js`'s terrain-shadow overlay) read the actual decoded tile