Report rain in whole millimetres; lift tier spans out of the bars (#192)

Two fixes to how measurements read.

Rain goes back to millimetres, rendered as whole numbers — that is how rainfall
is reported, and a tenth of a millimetre is below what the source resolves.
Inches keep two decimals, since a whole inch of rain is a lot to round to.

The distribution strip's low-high span moves from inside each bar to just under
the average, above it. A column is ~38px on a phone, so the old "Max 62° / Min
17°" pill clipped — it had already been shrunk to 8px to cope, and on a
high-rainfall city with longer values it lost both ends of the label, because the
text is centred and overflow-hidden. Above the bar it has the full column width
and needs no scrim to stay legible over nine tier colours.

The span is now bare numbers ("17-62"), since the average directly above it
carries the unit; repeating " g/m³" on both ends is what made it too wide in the
first place. It uses the same precision as that average, or the two lines
disagree ("52mm" over "12.7-136.91"). Sub-1 inch values drop their leading zero
so precip's ten columns still fit a phone.

With nothing inside the bars, the height floor drops from 30px to 14px: it only
has to keep a non-empty bucket visible now, so the heights encode frequency more
honestly. Category names get 2px of side padding, which stops long neighbours
("Light-Mod" beside "Moderate") reading as one word.

Verified at 390/412/1920 in both themes across all four calendar metrics and the
compare strips, in imperial and metric: nothing clips.

Claude-Session: https://claude.ai/code/session_013dRZmX9D3JEntfMKWMTWZ8

content.py

@@ -69,9 +69,9 @@ _TEMP_METRICS = {"tmax", "tmin", "feels"}
 F_COUNTRIES = frozenset({"US", "PR", "GU", "VI", "AS", "MP", "UM",
                          "BS", "BZ", "KY", "PW", "FM", "MH", "LR"})
 
-# One flag drives every measure: a °C page also gets cm and km/h. Mirrors the same
+# One flag drives every measure: a °C page also gets mm and km/h. Mirrors the same
 # decision in frontend/units.js.
-CM_PER_IN = 2.54
+MM_PER_IN = 25.4
 KMH_PER_MPH = 1.609344
 
 # The unit the current page renders temperatures in. None = °F, and also means
@@ -171,11 +171,13 @@ def _precip(v):
 
 
 def _precip_text(v) -> str:
-    """Two decimals either way — cm and inches are only 2.54x apart, so cm needs
+    """Whole millimetres — that is how rainfall is reported, and a tenth of a
-    the same resolution, and the source is hundredths of an inch."""
+    millimetre is below what the source resolves. Inches keep two decimals, since
+    a whole inch of rain is a lot to round to."""
     if v is None:
         return "—"
-    return f"{v * CM_PER_IN:.2f} cm" if _UNIT.get() == "C" else f"{v:.2f} in"
+    return (f"{_round_half_up(v * MM_PER_IN)} mm" if _UNIT.get() == "C"
+            else f"{v:.2f} in")
 
 
 def _wind(v):

tests/test_content.py

@@ -304,11 +304,11 @@ def test_units_default_to_the_citys_country(client):
     assert 'data-unit-default="F"' in us
 
 
-_PRECIP_SPAN = re.compile(r'<span class="precip" data-precip-in="([\d.]+)">([\d.]+) (in|cm)</span>')
+_PRECIP_SPAN = re.compile(r'<span class="precip" data-precip-in="([\d.]+)">([\d.]+) (in|mm)</span>')
 
 
 def test_precip_and_wind_follow_the_citys_unit(client):
-    # One toggle drives every measure: a °C page shows cm and km/h, not °C mixed
+    # One toggle drives every measure: a °C page shows mm and km/h, not °C mixed
     # with inches and mph. The data-* attribute stays imperial in both, so
     # climate.js converts from the same source of truth it does for temperature.
     gb = client.get(f"{B}/climate/{SLUG}/records").text            # London, GB
@@ -317,12 +317,12 @@ def test_precip_and_wind_follow_the_citys_unit(client):
     gb_p, us_p = _PRECIP_SPAN.findall(gb), _PRECIP_SPAN.findall(us)
     assert gb_p and us_p
 
-    assert {u for _raw, _shown, u in gb_p} == {"cm"}
+    assert {u for _raw, _shown, u in gb_p} == {"mm"}
     assert {u for _raw, _shown, u in us_p} == {"in"}
 
     # The conversion is real, not just a relabel.
     for raw, shown, _u in gb_p:
-        assert abs(float(shown) - float(raw) * 2.54) < 0.005, (raw, shown)
+        assert abs(float(shown) - float(raw) * 25.4) < 0.51, (raw, shown)
     # On an imperial page the attribute and the text agree.
     for raw, shown, _u in us_p:
         assert abs(float(shown) - float(raw)) < 0.005, (raw, shown)
@@ -348,14 +348,14 @@ def test_wind_and_humidity_render_per_unit_system():
 
 
 def test_precip_precision_differs_by_unit():
-    # cm and inches are only 2.54x apart, so cm keeps two decimals — at one
+    # Rainfall is reported in whole millimetres; a tenth of a mm is below what
-    # decimal every light-rain day would collapse to 0.0 or 0.1 cm.
+    # the source resolves. Inches keep two decimals.
     import content
     with content._unit_scope("F"):
         assert content._precip_text(0.04) == "0.04 in"
     with content._unit_scope("C"):
-        assert content._precip_text(0.04) == "0.10 cm"
+        assert content._precip_text(0.04) == "1 mm"
-        assert content._precip_text(1.81) == "4.60 cm"
+        assert content._precip_text(1.81) == "46 mm"
 
 
 def test_measure_conversion_constants_match_the_frontend():
@@ -366,9 +366,9 @@ def test_measure_conversion_constants_match_the_frontend():
                             os.pardir, "frontend", "units.js")
     with open(units_js, encoding="utf-8") as f:
         src = f.read()
-    cm = float(re.search(r"CM_PER_IN\s*=\s*([\d.]+)", src).group(1))
+    mm = float(re.search(r"MM_PER_IN\s*=\s*([\d.]+)", src).group(1))
     kmh = float(re.search(r"KMH_PER_MPH\s*=\s*([\d.]+)", src).group(1))
-    assert cm == content.CM_PER_IN
+    assert mm == content.MM_PER_IN
     assert kmh == content.KMH_PER_MPH