fix(api): persist time_format_default + chart_palette in settings

The Settings struct was missing TimeFormatDefault (Phase-45/22) and
ChartPalette (Phase-45/23) fields. When the Appearance settings page
PUT them, json.NewDecoder silently dropped the unknown JSON keys, the
Upsert never wrote the rows, and the next GET returned the old value -
so the radio cards appeared inert when clicked.

Wires both fields through the four canonical settings layers:
- models.Settings struct (with phase-tagged doc comments)
- settingsDefaults() (relative + cb_safe)
- applySettingsRow() switch cases
- Upsert() textRows
- Update() handler validation (rejects values outside the typed enum)

No migration needed: the settings key/value table is forward-
compatible and will create the rows on first Upsert.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: atulmgupta

internal/api/settings_handler.go

@@ -96,6 +96,16 @@ func (h *SettingsHandler) Update(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusBadRequest, "ui_density must be 'compact', 'comfortable', or 'spacious'")
 		return
 	}
+	validTimeFormat := map[string]bool{"relative": true, "absolute": true}
+	if s.TimeFormatDefault != "" && !validTimeFormat[s.TimeFormatDefault] {
+		writeError(w, http.StatusBadRequest, "time_format_default must be 'relative' or 'absolute'")
+		return
+	}
+	validChartPalette := map[string]bool{"cb_safe": true, "neon": true}
+	if s.ChartPalette != "" && !validChartPalette[s.ChartPalette] {
+		writeError(w, http.StatusBadRequest, "chart_palette must be 'cb_safe' or 'neon'")
+		return
+	}
 
 	// Record gas price change in history if price or unit changed
 	if s.GasPricePerUnit > 0 {

internal/database/settings_repo.go

@@ -65,6 +65,8 @@ func settingsDefaults() *models.Settings {
 		TabBadgeEnabled:      true,
 		CriticalFlashEnabled: true,
 		UIDensity:            "comfortable",
+		TimeFormatDefault:    "relative",
+		ChartPalette:         "cb_safe",
 	}
 }
 
@@ -209,6 +211,14 @@ func applySettingsRow(s *models.Settings, key, _ string, vText *string, vNum *fl
 		if vText != nil {
 			s.UIDensity = *vText
 		}
+	case "time_format_default":
+		if vText != nil {
+			s.TimeFormatDefault = *vText
+		}
+	case "chart_palette":
+		if vText != nil {
+			s.ChartPalette = *vText
+		}
 	}
 }
 
@@ -282,6 +292,8 @@ func (r *SettingsRepo) Upsert(ctx context.Context, s *models.Settings) error {
 		{"tz_display_default", s.TzDisplayDefault},
 		{"timezone_user", s.TimezoneUser},
 		{"ui_density", s.UIDensity},
+		{"time_format_default", s.TimeFormatDefault},
+		{"chart_palette", s.ChartPalette},
 	}
 	numRows := []rowNum{
 		{"base_cost_per_kwh", s.BaseCostPerKWh},

internal/models/system.go

@@ -367,6 +367,20 @@ type Settings struct {
 	// "comfortable" so existing users see no visual change.
 	// (Phase 40 / 44.)
 	UIDensity string `json:"ui_density"`
+	// TimeFormatDefault selects the default rendering of <TimeStamp>
+	// when no explicit `format` prop is set. One of "relative"
+	// (e.g. "2h ago", best for activity feeds) or "absolute"
+	// (e.g. "Nov 12, 13:42", best for trip planning + event
+	// correlation). The alternate format is always shown on hover.
+	// Defaults to "relative". (Phase 45 / 22.)
+	TimeFormatDefault string `json:"time_format_default"`
+	// ChartPalette selects the colour ramp used by multi-series
+	// charts. One of "cb_safe" (Okabe-Ito, distinguishable for all
+	// CVD types — the default) or "neon" (bright cyan/magenta legacy
+	// palette). Drives the reactive `useChartPalette()` hook so
+	// consumers re-render with the new colours when the user toggles.
+	// (Phase 45 / 23.)
+	ChartPalette string `json:"chart_palette"`
 }
 
 // Embedding mirrors the post-migration `embeddings` schema (pgvector-backed).