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>

Dockerfile

@@ -47,8 +47,11 @@ RUN go build -o /out/mavend    ./cmd/mavend    && \
     go build -o /out/mavcaldav ./cmd/mavcaldav
 
 FROM debian:trixie-slim AS runtime
+# tzdata so the TZ env (set in compose) resolves — otherwise Go can't load the
+# zone and time.Now() stays UTC, and mavend answers clock/date queries and
+# evaluates quiet-hours in UTC.
 RUN apt-get update && apt-get install -y --no-install-recommends \
-      ca-certificates libvulkan1 mesa-vulkan-drivers libgomp1 && \
+      ca-certificates libvulkan1 mesa-vulkan-drivers libgomp1 tzdata && \
     rm -rf /var/lib/apt/lists/*
 
 # runtime native libs: whisper/ggml (incl. vulkan) are real files in deps/lib.