feat(ui): warm light theme and pre-mixed status fills

- Light mode moves to Claude's ivory/terracotta family: page #e8e5da, cards
  #f5f3ec (a notch below Claude's ivory, no pure-white step), warm
  borders and card shadow; dark mode is untouched.
- The surface ramp rises in lightness with its index in BOTH themes, so a
  card sits brighter than the page and every hover lifts instead of dims.
- Light --accent #a34a28 (the brand #cc785c is 3.1:1 on ivory); --fg-muted
  held at >=4.5:1; new --syn-* syntax tokens, dark keeps its pastels.
- Status/accent fills (bg-*/N, border-*/N) compile to color-mix against
  --tint-base instead of rgb(/a), so a chip is one colour on every surface.
- Hardcoded dark-only hovers (hover:text-white, hover:bg-white/5) replaced
  by tokens in DateTimePicker and SessionDrillIn.

client/src/components/DateTimePicker.tsx

@@ -174,7 +174,7 @@ export function DateTimePicker({
               ? "bg-accent text-white font-medium"
               : isToday
                 ? "bg-surface-3 text-accent font-medium"
-                : "hover:bg-surface-2 text-fg-secondary hover:text-white"
+                : "hover:bg-surface-2 text-fg-secondary hover:text-fg-primary"
           }`}
       >
         {i}
@@ -201,7 +201,10 @@ export function DateTimePicker({
           {dateObj ? formatDisplay(dateObj) : placeholder}
         </span>
         {dateObj && (
-          <X className="w-3 h-3 text-fg-muted hover:text-white shrink-0" onClick={clearValue} />
+          <X
+            className="w-3 h-3 text-fg-muted hover:text-fg-primary shrink-0"
+            onClick={clearValue}
+          />
         )}
       </button>
 
@@ -216,7 +219,7 @@ export function DateTimePicker({
               onClick={() =>
                 setViewDate(new Date(viewDate.getFullYear(), viewDate.getMonth() - 1, 1))
               }
-              className="p-1 hover:bg-surface-2 rounded text-fg-secondary hover:text-white"
+              className="p-1 hover:bg-surface-2 rounded text-fg-secondary hover:text-fg-primary"
             >
               <ChevronLeft className="w-4 h-4" />
             </button>
@@ -228,7 +231,7 @@ export function DateTimePicker({
               onClick={() =>
                 setViewDate(new Date(viewDate.getFullYear(), viewDate.getMonth() + 1, 1))
               }
-              className="p-1 hover:bg-surface-2 rounded text-fg-secondary hover:text-white"
+              className="p-1 hover:bg-surface-2 rounded text-fg-secondary hover:text-fg-primary"
             >
               <ChevronRight className="w-4 h-4" />
             </button>