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/components/assistant/ChatMessage.tsx

@@ -71,7 +71,7 @@ export function ChatMessage({ role, content, suggestedFlows, imageUrls, actionCo
         {/* Suggested flows (assistant only) */}
         {role === 'assistant' && suggestedFlows && suggestedFlows.length > 0 && (
           <div className="space-y-1.5">
-            <span className="font-sans text-[0.625rem] uppercase tracking-widest text-muted-foreground">
+            <span className="text-[0.625rem] uppercase tracking-widest text-muted-foreground">
               Related Flows
             </span>
             {suggestedFlows.map(flow => (