feat(theme): dark/light mode with a Radix Colors-based palette

Adds a working Dark/Light toggle (next to the language switcher, same row
as EN/VI) and re-themes the whole dashboard, not just the handful of
components that already used semantic tokens.

- Tailwind darkMode:"class" + CSS-variable color tokens (client/src/index.css,
  tailwind.config.js): surface.0-5, border/border-light, accent/accent-hover,
  fg.primary/secondary/muted, status.success/danger/warning. One class flip
  on <html> re-themes everything — no per-element dark: variant pairs.
- useTheme() hook: localStorage-persisted, defaults to dark, no
  prefers-color-scheme fallback (client/src/hooks/useTheme.ts).
- Mechanical, table-driven migration (scripts/migrate-color-tokens.mjs,
  scripts/tokenize-status-colors.mjs, scripts/darken-status-colors.mjs) of
  every raw neutral/gray/slate + emerald/red/amber Tailwind utility across
  client/src onto the new tokens, so every badge/button/component pulls the
  same shade per status/role instead of each picking its own.
- Palette values are the literal Radix Colors (radix-ui.com/colors) scale
  constants — slate/blue/green/red/amber steps 1-12 — adopted after three
  rounds of hand-picked values that kept overshooting (flat, then too dark,
  then glaring); see docs/superpowers/specs/2026-07-31-color-redesign-
  dark-light-mode-design.md for the full history and role mapping.
- PipelineMap: done/current/failed/passed-no-evidence/detected share one
  visual language (border + text + translucent wash of the same status
  color); `current` alone stays a solid accent fill, the one state that
  gets to look bolder ("you are here").
- LaneCard: removed the stage/kind/auto-stage chips that duplicated the
  Workspace lane-detail header already showing them.

Categorical/decorative hues (violet, indigo, cyan, teal, sky, rose, pink,
orange, yellow, and blue where it plays a role-coloring part e.g. message
bubbles) are deliberately out of scope — collapsing those onto shared
tokens would erase the distinction between different kinds of thing, not
a status.

client/src/components/DateTimePicker.tsx

@@ -180,7 +180,7 @@ export function DateTimePicker({
               ? "bg-accent text-white font-medium"
               : isToday
                 ? "bg-surface-3 text-accent font-medium"
-                : "hover:bg-surface-2 text-gray-300 hover:text-white"
+                : "hover:bg-surface-2 text-fg-secondary hover:text-white"
           }`}
       >
         {i}
@@ -202,12 +202,12 @@ export function DateTimePicker({
         title={title}
         className={`flex items-center gap-2 bg-surface-2 border ${isOpen ? "border-accent" : "border-border"} rounded px-2 py-1.5 min-w-[150px] text-xs focus:outline-none focus:border-accent transition-colors w-full text-left`}
       >
-        <Calendar className="w-3.5 h-3.5 text-gray-400 shrink-0" />
-        <span className={`flex-1 truncate ${!dateObj ? "text-gray-500" : "text-gray-200"}`}>
+        <Calendar className="w-3.5 h-3.5 text-fg-secondary shrink-0" />
+        <span className={`flex-1 truncate ${!dateObj ? "text-fg-muted" : "text-fg-secondary"}`}>
           {dateObj ? formatDisplay(dateObj) : placeholder}
         </span>
         {dateObj && (
-          <X className="w-3 h-3 text-gray-500 hover:text-white shrink-0" onClick={clearValue} />
+          <X className="w-3 h-3 text-fg-muted hover:text-white shrink-0" onClick={clearValue} />
         )}
       </button>
 
@@ -222,11 +222,11 @@ export function DateTimePicker({
               onClick={() =>
                 setViewDate(new Date(viewDate.getFullYear(), viewDate.getMonth() - 1, 1))
               }
-              className="p-1 hover:bg-surface-2 rounded text-gray-400 hover:text-white"
+              className="p-1 hover:bg-surface-2 rounded text-fg-secondary hover:text-white"
             >
               <ChevronLeft className="w-4 h-4" />
             </button>
-            <span className="text-xs font-medium text-gray-200">
+            <span className="text-xs font-medium text-fg-secondary">
               {viewDate.toLocaleString(undefined, { month: "long", year: "numeric" })}
             </span>
             <button
@@ -234,7 +234,7 @@ export function DateTimePicker({
               onClick={() =>
                 setViewDate(new Date(viewDate.getFullYear(), viewDate.getMonth() + 1, 1))
               }
-              className="p-1 hover:bg-surface-2 rounded text-gray-400 hover:text-white"
+              className="p-1 hover:bg-surface-2 rounded text-fg-secondary hover:text-white"
             >
               <ChevronRight className="w-4 h-4" />
             </button>
@@ -244,7 +244,7 @@ export function DateTimePicker({
           <div>
             <div className="grid grid-cols-7 gap-1 mb-1">
               {["Su", "Mo", "Tu", "We", "Th", "Fr", "Sa"].map((day) => (
-                <div key={day} className="w-6 text-center text-[10px] font-medium text-gray-500">
+                <div key={day} className="w-6 text-center text-[10px] font-medium text-fg-muted">
                   {day}
                 </div>
               ))}
@@ -254,7 +254,7 @@ export function DateTimePicker({
 
           {/* Time Picker */}
           <div className="pt-3 border-t border-border flex items-center justify-between">
-            <div className="flex items-center gap-1.5 text-gray-400">
+            <div className="flex items-center gap-1.5 text-fg-secondary">
               <Clock className="w-3.5 h-3.5" />
               <span className="text-[11px] font-medium">Time</span>
             </div>
@@ -262,7 +262,7 @@ export function DateTimePicker({
               type="time"
               value={timeValue}
               onChange={handleTimeChange}
-              className="bg-surface-2 border border-border rounded px-2 py-1 text-xs text-gray-200 focus:outline-none focus:border-accent w-[85px] time-input-custom"
+              className="bg-surface-2 border border-border rounded px-2 py-1 text-xs text-fg-secondary focus:outline-none focus:border-accent w-[85px] time-input-custom"
             />
           </div>
         </div>