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.

README.md

@@ -78,9 +78,10 @@ Global:
 | `esc` | Go back |
 | `r` | Refresh |
 | `f` | Cycle filter |
-| `S` | Statistics |
 | `q` | Quit |
 
+The sections, in `tab` order: Incidents · Alerts · Stats · Archived · Schedule · Users.
+
 Incidents section:
 
 | Key | Action |
@@ -108,6 +109,12 @@ Alerts section (read-only):
 | `f` | Cycle: firing → resolved → all → archived |
 | `i` | In detail: jump to the alert's incident |
 
+Stats section:
+
+| Key | Action |
+|-----|--------|
+| `j` / `k`, `pgup` / `pgdn` | Scroll |
+
 Schedule section:
 
 | Key | Action |