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/TaskLane.tsx

@@ -482,7 +482,7 @@ export function TaskLane({ questions, actions, sessionId, onSubmit, onClose, loa
                             Cancel
                           </button>
                         </div>
-                        <span className="text-[0.625rem] text-muted-foreground/70 tabular-nums">
+                        <span className="text-[0.625rem] text-muted-foreground tabular-nums">
                           ⏎ submit · ⇧⏎ newline
                         </span>
                       </div>
@@ -675,7 +675,7 @@ export function TaskLane({ questions, actions, sessionId, onSubmit, onClose, loa
                             Cancel
                           </button>
                         </div>
-                        <span className="text-[0.625rem] text-muted-foreground/70 tabular-nums">
+                        <span className="text-[0.625rem] text-muted-foreground tabular-nums">
                           ⏎ submit · ⇧⏎ newline
                         </span>
                       </div>