fix(common): expose /etc/timezone so flatpak apps stop defaulting to UTC

Flatpak detects the sandbox timezone from /etc/timezone, not the
/etc/localtime symlink NixOS creates by default. Without it every
flatpak app (Telegram included) silently falls back to UTC, showing
message timestamps 2h off from local time.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UJqEmY1y3AYX3JoX4Y6b21

common.nix

@@ -1,4 +1,4 @@
-{ pkgs, ... }:
+{ pkgs, config, ... }:
 
 # Shared base for all hosts: user, SSH hardening, nix settings, packages.
 {
@@ -99,6 +99,8 @@
 
   # ---- Locale / firewall base ----
   time.timeZone = "Europe/Berlin";
+  # Flatpak needs /etc/timezone, falls back to UTC if not set.
+  environment.etc."timezone".text = config.time.timeZone;
   i18n.defaultLocale = "en_US.UTF-8";
   console.keyMap = "de";