fix: local timezone for replies + quiet-hours; guard presence "ago" overflow

Timezone: the container ran in UTC, so mavend answered clock/date queries
(voice.go replySystem) and evaluated quiet-hours (gather.go) in UTC. Fixed at
the root — process TZ — rather than per-call: TZ=Europe/Samara in compose +
tzdata in the image (debian-slim strips it, without which Go ignores TZ and
stays UTC). One knob fixes replies and quiet-hours for every daemon; change the
zone in compose.

Overflow: the dash "ago" helper ran time.Since on a zero timestamp (no presence
yet / fresh db), saturating to ~292y and rendering "2562047h47m…". Guard zero →
"never".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.yml

@@ -15,6 +15,10 @@ x-image: &image
   build: .
   pull_policy: never   # only ever the locally-built image
   restart: unless-stopped
+  # local time for clock/date replies AND quiet-hours evaluation. Change to
+  # your zone; needs tzdata in the image (installed in the Dockerfile).
+  environment:
+    - TZ=Europe/Samara
 
 services:
   mavend: