style: theme-aware token pass for dark-mode surfaces (Phase 18)

Re-point neutral page surfaces, play-chip, and default popover from constant brand tokens to theme-aware aliases defined twice in deepdrft-tokens.css. Decorative navy/green sections and bespoke dark-glass panels untouched. Appbar-navy symptom deferred (palette C#, out of CSS scope).

DeepDrftPublic.Client/Layout/DeepDrftFooter.razor.css

@@ -3,7 +3,7 @@
        WaveformVisualizer backdrop (z-index:0), keeping footer text fully legible. */
     position: relative;
     z-index: 1;
-    background: var(--deepdrft-white);
+    background: var(--deepdrft-page-surface);
     border-top: 1px solid var(--deepdrft-border);
     padding: 3rem;
     display: flex;
@@ -22,7 +22,7 @@
     font-family: var(--deepdrft-font-display);
     font-size: 1.5rem;
     font-weight: 400;
-    color: var(--deepdrft-navy);
+    color: var(--deepdrft-page-text);
 }
 
 .deepdrft-footer-logo span {
@@ -44,19 +44,19 @@
     font-size: 0.62rem;
     letter-spacing: 0.18em;
     text-transform: uppercase;
-    color: var(--deepdrft-muted);
+    color: var(--deepdrft-page-text-muted);
     text-decoration: none;
     transition: color 0.2s;
 }
 
 .deepdrft-footer-links a:hover,
-.deepdrft-footer-links button:hover { color: var(--deepdrft-navy); }
+.deepdrft-footer-links button:hover { color: var(--deepdrft-page-text); }
 
 .deepdrft-footer-copy {
     font-family: var(--deepdrft-font-mono);
     font-size: 0.58rem;
     letter-spacing: 0.12em;
-    color: var(--deepdrft-muted);
+    color: var(--deepdrft-page-text-muted);
 }
 
 /* PRIVACY trigger — reset button chrome so it reads as a link, not a button element.