fix(gui): theme the rotating time cursor (it rendered black)

The cursor line and two sub-interval traces set their colour to
"var(--accent)" / "var(--good)". Plotly writes shape and trace colours
straight into the SVG, and SVG presentation attributes do NOT resolve CSS
custom properties — so the value was invalid and fell back to black,
leaving the time cursor near-invisible against the dark spectrogram.

Use the literal --accent/--good values per theme, matching how the rest of
this file already themes its plot colours (`dark ? … : …`).

Also fixes the same defect in the sub-interval Coherence and Cross-Power
traces, which are the same bug 60 lines away and were equally black.

Only the Plotly colours were affected; the remaining var(--…) uses in this
file are on real DOM elements, where CSS variables resolve correctly.

Verified in-browser: the cursor's rendered stroke is rgb(46,230,207) in
dark and rgb(10,141,128) in light — never black, and it follows the theme.

Author: matt-pharr

gui/web/src/components/tabs/RotatingTab.tsx

@@ -799,7 +799,11 @@ export default function RotatingTab({ machine }: { machine: string }) {
           x1: cursorMs,
           y1: 1,
           line: {
-            color: "var(--accent)", // bright turquoise
+            // Plotly writes this straight into the SVG `stroke`, which does NOT
+            // resolve CSS custom properties — "var(--accent)" was invalid there and
+            // fell back to black, leaving the cursor near-invisible in dark mode.
+            // Use the literal --accent value for each theme.
+            color: dark ? "#2ee6cf" : "#0a8d80",
             width: 2,
             dash: "dash" as const,
           },
@@ -848,7 +852,7 @@ export default function RotatingTab({ machine }: { machine: string }) {
         mode: "lines" as const,
         x: freqs,
         y: coh,
-        line: { color: "var(--good)", width: 1.5 },
+        line: { color: dark ? "#54e08a" : "#1f9d57", width: 1.5 }, // --good, literal (see above)
         name: "Coherence",
         xaxis: "x",
         yaxis: "y",
@@ -872,7 +876,7 @@ export default function RotatingTab({ machine }: { machine: string }) {
         mode: "lines" as const,
         x: freqs,
         y: power,
-        line: { color: "var(--accent)", width: 1.5 },
+        line: { color: dark ? "#2ee6cf" : "#0a8d80", width: 1.5 }, // --accent, literal (see above)
         name: "Cross-Power",
         xaxis: "x",
         yaxis: "y3",