fix(card): productive titles render at 15 px for every timezone (#24)

Two cuts together keep the productive-time and productive-weekday
titles at the same 15 px the rest of the gallery uses:

1. utcOffsetLabel is now compact. Integer offsets drop the '.00'
   padding ("UTC+7" vs the old "UTC+7.00") and non-integer offsets use
   the colon form ("UTC+5:30", "UTC+5:45"). Saves 3 chars for the
   common integer case, so "Commits by Hour (last year, UTC+7)" lands
   at 34 chars — inside the 15 px budget.

2. weekdayTitle no longer embeds the UTC label. Day-of-week aggregates
   by whole days; clock precision isn't informative there, and
   dropping it shortens the title to 30 chars so the full 15 px lands
   for every timezone.

Quarter-hour-zone users (Kathmandu UTC+5:45) see the hour title drop
to 14 px — 37 chars still exceeds the budget — but that's a rare
case and only 1 px off.

TestUTCOffsetLabel updated to the new format. TestFitTitleFontSize
pinned the new titles.

docs/design-guidelines.md

@@ -70,7 +70,7 @@ When there's **exactly one slice** (one language at 100%), the renderer emits tw
 | Bar fill | `theme.Accent` for the peak bar; `mixHex(Background, Accent, 0.55)` dim for the rest so the busiest period reads at a glance |
 | Y-axis ticks | `niceTicks(max, 5)` — 1/2/5 × 10^k ladder. `last = ceil(max/step) × step` so `yMax ≥ dataMax` always (bars can't poke above chartH into the title) |
 | Axis caption | "hour of day" bottom-center on productive-time; weekday / by-year omit the caption since the x labels are self-describing |
-| Title format | `Commits by Hour (<window>, UTC±N.NN)` / `Commits by Weekday (<window>, UTC±N.NN)` / `Contributions by Year` |
+| Title format | `Commits by Hour (<window>, UTC±H[:MM])` / `Commits by Weekday (<window>)` — weekday drops the UTC so the title always fits at 15 px / `Contributions by Year` |
 | Hover | `<title>HH:00 — N commits</title>` / `<title>Mon — N commits</title>` / `<title>YYYY — N commits</title>` |
 
 ## Heatmap card (contributions-heatmap)