fix(ui): promote modal overlay to token, lift modal above grain, dedupe FilesPage drop-zone

- Add --color-overlay-dim token to app.css @theme (rgba(15,14,12,0.7) dark, rgba(26,24,22,0.55) light); mirror as --overlay-dim in design-system/colors_and_type.css.
- MorningTriageModal: replace inlined rgba scrim with var(--color-overlay-dim), drop backdrop-blur-sm (brand opposes glassmorphism by default), bump container to z-[60] so it sits above the .grain z-50 overlay.
- FilesPage: collapse two near-identical drop-zone blocks into one bordered region with conditional inner content; preserves isDragOver hover behaviour and aria region.

src/app.css

@@ -69,6 +69,10 @@
   --color-danger: #e87171;
   --color-warning: #e8c86e;
 
+  /* Overlays — used by modal scrims. Derived from --color-bg #0f0e0c at
+     0.7 alpha so the dim sits on the brand neutral, not pure black. */
+  --color-overlay-dim: rgba(15, 14, 12, 0.7);
+
   /* Layout */
   --color-sidebar: #13120f;
   --color-nav-active: #201e1a;
@@ -136,6 +140,10 @@
   --color-sidebar: #f5f2ed;
   --color-nav-active: #eae6e0;
   --color-hover: #ede9e3;
+
+  /* Warm dim for modal scrims in light theme. Lower alpha so the cream
+     surface still reads through softly. */
+  --color-overlay-dim: rgba(26, 24, 22, 0.55);
 }
 
 :root[data-theme="light"] ::-webkit-scrollbar-thumb {