feat(ui): terminal, charts and low-contrast text follow the selected theme

- The lane terminal (xterm) takes its palette from the selected DARK theme —
  it stays dark in light mode, as a terminal should — and repaints live when
  the theme changes (`client/src/lib/terminalTheme.ts`).
- Charts on the Dashboard and Workflows pages draw their grid, axes, labels
  and tooltips from theme tokens instead of hardcoded near-black/white, and
  the activity heatmap ramps from the page surface to the accent, so it reads
  on every theme instead of going near-black on light ones.
- Status/brand text that only worked on dark backgrounds
  (`text-*-300/400`) gets a darker light-mode shade (`text-*-700
  dark:text-*-300`) across sessions, conversations, workflows, config and
  settings. Categorical chart colours are kept.

client/src/components/event-views/primitives.tsx

@@ -289,7 +289,7 @@ function HunkView({ hunk }: { hunk: DiffHunk }) {
   let newLine = hunk.newStart;
   return (
     <div>
-      <div className="px-3 py-1 text-[10px] text-cyan-300 bg-cyan-500/10 border-y border-cyan-500/20 font-mono">
+      <div className="px-3 py-1 text-[10px] text-cyan-700 dark:text-cyan-300 bg-cyan-500/10 border-y border-cyan-500/20 font-mono">
         @@ -{hunk.oldStart},{hunk.oldLines} +{hunk.newStart},{hunk.newLines} @@
       </div>
       <table className="w-full border-collapse">
@@ -301,7 +301,7 @@ function HunkView({ hunk }: { hunk: DiffHunk }) {
             const showNew = kind !== "remove";
             const rowBg =
               kind === "add"
-                ? "bg-green-500/10 text-green-200"
+                ? "bg-green-500/10 text-green-800 dark:text-green-200"
                 : kind === "remove"
                   ? "bg-status-danger/10 text-status-danger"
                   : "text-fg-secondary";
@@ -372,7 +372,7 @@ function ValueCell({ value }: { value: unknown }) {
       <span
         className={`inline-block px-2 py-0.5 rounded border text-[11px] font-mono ${
           value
-            ? "text-green-400 border-green-500/30 bg-green-500/10"
+            ? "text-green-700 dark:text-green-400 border-green-500/30 bg-green-500/10"
             : "text-fg-secondary border-border-light/30 bg-surface-4/10"
         }`}
       >
@@ -445,7 +445,7 @@ export function MatchList({ matches }: { matches: GrepMatch[] }) {
     <ul className="divide-y divide-border border border-border rounded overflow-hidden text-[11px] max-h-80 overflow-y-auto font-mono">
       {matches.map((m, i) => (
         <li key={i} className="px-3 py-1 text-fg-secondary break-all">
-          {m.file && <span className="text-cyan-300">{m.file}</span>}
+          {m.file && <span className="text-cyan-700 dark:text-cyan-300">{m.file}</span>}
           {m.line != null && <span className="text-fg-muted">:{m.line}</span>}
           {m.text && <span className="text-fg-secondary">: {m.text}</span>}
         </li>