fix(session-ui): restore blue user bubbles with grey pending steers - #57001

Why

#56961 made every user bubble grey. Grey everywhere makes your own messages hard to pick out of the timeline, and a pending steer looked almost the same as a delivered message. It only differed by 60% opacity.

What Changes

• Delivered messages: every user bubble now uses the old non-worktree (local session) blue in both session kinds. Dark is --v2-blue-1200 with --v2-blue-300 text. Light is --v2-blue-100 with --v2-blue-700 text.
• No workspace accent: worktree sessions no longer get the bright bg-accent bubble. They use the same dark blue.
• Pending steers: a pending steer is grey again (--v2-background-bg-layer-02, --v2-text-text-base), with no opacity dimming. The @ mention prefix is --v2-blue-500 on delivered bubbles and follows the text colour on pending ones.
• Delivery animation: going pending is instant. Delivery fades grey to blue over 150ms ease-out, and reduced motion makes that instant.
• Kept from fix(session-ui): use one grey bubble for user messages #56961: the pending steer's meta row (Pending, agent, model, time, plus its actions) stays visible without hover. The bubble keeps the 10px radius, and data-local-session/data-workspace-session stay removed.

Verification

• new-session expects the blue bubble for both the worktree and main-root sessions.
• session-queue expects a grey pending steer with no transition, its Pending row visible without hover, and a blue bubble after delivery through a background-color, color transition that becomes 0s under reduced motion.
• I left the e2e and visual checks to CI. I did not run them locally.