sky form TZ: render-readonly + drop #id_nf_tz_hint; placeholder absorbs the auto-detected hint copy — TDD

A user-typed TZ override fed through schedulePreview's `if (tz) params.set('tz', tz)` path made PySwiss compute the chart against a TZ that didn't match the lat/lon, so a partial edit (e.g. "America/New_Yo|") returned HTTP 400. Mirror the lat/lon convention: tz field gets readonly + tabindex:-1 across all three sky contexts (Dashsky sky.html, in-room PICK SKY _sky_overlay.html, My Sky applet _applet-my-sky.html). Auto-population still works because the JS writes via .value rather than via user input. The <small id="id_nf_tz_hint"> "Auto-detected from coordinates." line is removed; that copy now lives on the <input>'s placeholder so an empty TZ field self-explains. JS purges every tzHint reference (const declaration + 4 .textContent writes per file × 3 files).

SkyViewTest.test_tz_input_is_readonly_and_carries_auto_detect_placeholder pins the rendered Dashsky markup: id_nf_tz carries `readonly`, the placeholder is "auto-detected from coordinates", and `id="id_nf_tz_hint"` no longer appears anywhere. Existing MySkyTimezoneRefreshTest still passes — it asserts the field auto-fills via JS, which still works on a readonly input.

Code architected by Disco DeDisco <discodedisco@outlook.com>
Git commit message Co-Authored-By:
Claude Sonnet 4.6 <noreply@anthropic.com>

src/templates/apps/gameboard/_partials/_sky_overlay.html

@@ -71,8 +71,8 @@
                         <div class="sky-field">
                             <label for="id_nf_tz">Timezone</label>
                             <input id="id_nf_tz" name="tz" type="text"
-                                   placeholder="auto-detected from location">
-                            <small id="id_nf_tz_hint"></small>
+                                   placeholder="auto-detected from coordinates"
+                                   readonly tabindex="-1">
                         </div>
 
                     </form>
@@ -122,7 +122,6 @@
   const latInput     = document.getElementById('id_nf_lat');
   const lonInput     = document.getElementById('id_nf_lon');
   const tzInput      = document.getElementById('id_nf_tz');
-  const tzHint       = document.getElementById('id_nf_tz_hint');
   const suggestions  = document.getElementById('id_nf_suggestions');
 
   const PREVIEW_URL  = overlay.dataset.previewUrl;
@@ -166,7 +165,7 @@
     if (data.place) placeInput.value = data.place;
     if (data.lat)   latInput.value   = data.lat;
     if (data.lon)   lonInput.value   = data.lon;
-    if (data.tz)    { tzInput.value  = data.tz; tzHint.textContent = 'Auto-detected from coordinates.'; }
+    if (data.tz)    { tzInput.value  = data.tz; }
   }
 
   // ── Open / Close ──────────────────────────────────────────────────────────
@@ -251,7 +250,6 @@
     latInput.value   = parseFloat(place.lat).toFixed(4);
     lonInput.value   = parseFloat(place.lon).toFixed(4);
     tzInput.value    = '';
-    tzHint.textContent = '';
     hideSuggestions();
     _saveForm();
     schedulePreview();
@@ -270,7 +268,6 @@
         latInput.value = pos.coords.latitude.toFixed(4);
         lonInput.value = pos.coords.longitude.toFixed(4);
         tzInput.value  = '';
-        tzHint.textContent = '';
         fetch(`https://nominatim.openstreetmap.org/reverse?format=json&lat=${latInput.value}&lon=${lonInput.value}`, {
           headers: { 'User-Agent': USER_AGENT },
         })
@@ -335,7 +332,6 @@
         // Back-fill timezone field from proxy response (first render)
         if (!tzInput.value && data.timezone) {
           tzInput.value = data.timezone;
-          tzHint.textContent = 'Auto-detected from coordinates.';
         }
 
         setStatus('');
@@ -429,7 +425,6 @@
         latInput.value = '';
         lonInput.value = '';
         tzInput.value  = '';
-        tzHint.textContent = '';
         _lastChartData = null;
         confirmBtn.disabled = true;
         setStatus('');