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/lib/components/MorningTriageModal.svelte

@@ -258,14 +258,14 @@
 
 {#if open}
   <!--
-    Pure-black-white taste fix: replaced `bg-black/50` overlay with the brand
-    deep-neutral `#1a1816` at 50% alpha. TODO: promote this colour to a
-    `--color-overlay` token in app.css so the value isn't inlined per-modal.
+    Overlay sits on `--color-overlay-dim` (defined in app.css @theme), warm
+    deep-neutral derived from --color-bg. No backdrop-blur: the brand opposes
+    glassmorphism as a default. z-[60] sits above the .grain overlay (z-50).
   -->
   <div
     bind:this={dialogEl}
-    class="fixed inset-0 z-50 flex items-center justify-center backdrop-blur-sm animate-fade-in"
-    style="background: rgba(26, 24, 22, 0.5)"
+    class="fixed inset-0 z-[60] flex items-center justify-center animate-fade-in"
+    style="background: var(--color-overlay-dim)"
     role="dialog"
     aria-modal="true"
     aria-labelledby="triage-title"