[FEATURE]: Floating input navigation rail for session panel

[DevDengChao]

• I have verified this feature I'm about to request hasn't been suggested before.

---

Add a floating navigation rail for user messages in the session panel. Each user message gets a horizontal bar whose width encodes content length (short messages = narrow bars, long messages = wide bars). Hovering a bar shows a preview popup with truncated text, image thumbnails, or file icons.

Benefits:

• Quick visual overview of message lengths in long sessions
• Fast navigation between user messages
• Preview message content without scrolling
• Non-intrusive design that only appears on desktop

Proposed behavior:

• Controlled by showInputNav setting (default: false) in Settings > Advanced
• Only visible on non-mobile screens (768px+)
• Bar width proportional to message text length
• HoverCard preview showing text, images, and files

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: Persistent right-side message navigation sidebar in the desktop app session view #32999: Persistent right-side message navigation sidebar in the desktop app session view — very similar concept (sidebar listing user messages with click-to-jump, floating/persistent panel in the session view)
• [FEATURE]:User message quick-jump sidebar #32165: User message quick-jump sidebar — narrow sidebar listing only user prompts for quick navigation
• [FEATURE]:Replace Default Scrollbar with a Conversation Quick Navigation Bar Description #31633: Replace Default Scrollbar with a Conversation Quick Navigation Bar — right-edge nav bar with markers, hover preview, and click-to-jump
• [Feature]: scrollbar markers per message boundary for easier conversation navigation #37699: Scrollbar markers per message boundary for easier conversation navigation — visual markers on the scrollbar for each message boundary with preview