chore(session): bump keyboard hint contrast + drop redundant font-sans

Two small ergonomic fixes after the impeccable pass:

- TaskLane keyboard hints (⏎ submit · ⇧⏎ newline) under each open input
  were rendered at text-muted-foreground/70, just shy of legible at a
  glance. Drop the /70 opacity modifier so they read at full muted weight
  on first look without becoming visually loud.

- 12 sites across the session screen had explicit font-sans utilities,
  but the body default is already IBM Plex Sans (via --font-sans in
  index.css and Tailwind v4's default-sans binding). None of the call
  sites sit inside a font-heading or font-mono cascade, so every
  font-sans there was a no-op. Drop them. ConcludeSessionModal also had
  three "text-xs font-sans text-xs" triplets — drop both the redundant
  font-sans and the doubled text-xs in one pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/pages/AssistantChatPage.tsx

@@ -2128,7 +2128,7 @@ export default function AssistantChatPage() {
                   {showLogs && (
                     <div className="px-4 pb-1">
                       <div className="flex items-center justify-between mb-1">
-                        <span className="text-[0.625rem] uppercase tracking-wide text-muted-foreground font-sans">Paste logs or error output</span>
+                        <span className="text-[0.625rem] uppercase tracking-wide text-muted-foreground">Paste logs or error output</span>
                         <button type="button" onClick={() => { setShowLogs(false); setLogContent('') }} className="text-muted-foreground hover:text-foreground"><X size={14} /></button>
                       </div>
                       <textarea