feat(bar): timezone line in the clock tooltip (LATER item)

Hovering the clock now leads with the zone — "{:%Z (UTC%z)}" above the
calendar — on the generated bar and both summer whole-swaps (parity).
Under auto-timezone the existing tz-watch SIGUSR2 reload keeps it
showing the currently detected zone. %Z is the abbreviation (BST), not
the IANA name — strftime has no code for that, and it satisfies the
"(optional)" nicety.

Verified: V0; V1 — the rendered generated config carries the new
tooltip-format and both summer jsonc parse with it. Hover render
queued for hardware.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

modules/home/waybar.nix

@@ -150,7 +150,9 @@ let
     clock = {
       format = "{:%H:%M}";
       format-alt = "{:%A %d %B %Y}";
-      tooltip-format = "<tt><small>{calendar}</small></tt>";
+      # Zone line first (LATER nicety): under auto-timezone the tz-watch
+      # SIGUSR2 reload keeps it showing the currently *detected* zone.
+      tooltip-format = "{:%Z (UTC%z)}\n<tt><small>{calendar}</small></tt>";
     };
 
     # Active keyboard layout (per focused device) — only placed in