fix: simplify native session mode tooltip

[DylenTurnbull]

Description

Closes #413.

Replace the remote Native session mode question-mark popover with a concise browser tooltip on the mode value itself.

Key Changes:

• Removes the clickable ? control and its unused popover CSS.
• Shows Current mode reported by the active ACP harness. when hovering the native mode value.
• Keeps the value read-only and leaves all session behavior unchanged.

UI/Session Evidence:

• Surface(s): remote viewer Native session mode status.
• Workflow: open a live remote session and hover the displayed mode value.
• Before: a clickable ? appeared beside the value.
• After: no ? appears; the value itself exposes the concise tooltip.
• Safety or capability boundaries: no selector, mutation path, or permission behavior changed.
• Visual evidence: manually reviewed in the live Tailscale viewer.

Validation:

• git diff --check
• cargo build --release
• Live Tailscale viewer validation of the hover tooltip and removed ? control.
• No tests run; this is a focused HTML/CSS and tooltip-copy change.