feat!: stats as a section instead of an overlay

Stats was the one full-screen view reached by a key of its own rather
than by tab, and the interface was less coherent for it. It is now a
section sitting third, after Alerts, and behaves like every other one:
tab in, tab out, r to refresh.

Three things fall out of the move. It auto-refreshes for the first time
— the tick handler skips every non-dashboard mode, which is why the
overlay never updated while it was open. Its error path no longer forces
the queue back into view on a failed fetch, an assumption that only made
sense while stats floated above the dashboard. And first-visit loading
keys off a statsLoaded flag rather than slice emptiness, because the
three empty slices a quiet server returns are a real answer, not a
missing one; the loading placeholder is likewise suppressed once
something has been drawn, so a background refresh cannot blank the page
out from under whoever is reading it.

The S key is gone, and with it the ability to peek at statistics from an
open incident and land back on it. That round-trip was the only thing
statsReturnMode bought, and it was the whole reason stats needed a mode.

CLAUDE.md

@@ -73,6 +73,7 @@ go build -ldflags="-X main.version=v0.1.0" -o terdut-tui .
 ## Sections
 
 `Incidents` (the queue, and the default) · `Alerts` (raw read-only feed) ·
+`Stats` (MTTA/MTTR and alert frequency charts) ·
 `Archived` (archived incidents) · `Schedule` · `Users`
 
 ## Development stages