[FEATURE]: Add progressive scroll navigation buttons to TUI session view

[SOUMITRO-SAHA]

Feature hasn't been suggested before.

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

Describe the enhancement you want to request

What: Add two scroll navigation buttons (▲ / ▼) on the right side of the TUI session view to quickly jump between user messages and return to the bottom.

Why: In long sessions with many back-and-forth messages, scrolling manually to find previous user prompts is tedious. A quick navigation mechanism improves productivity when reviewing or editing prior prompts.

How it should work:

1. ▲ Up arrow — Jump to the previous user message progressively:

   • 1st click → jumps to the user message immediately above the current scroll position
   • 2nd click → jumps to the user message before that
   • Continues until reaching the first user message in the session

2. ▼ Down arrow — Scroll to the bottom of the session:

   • Appears only when the view is scrolled up (hidden when already at the bottom)
   • Clicking it resets the progressive navigation state

Benefits:

• Faster navigation in long sessions
• Reduces manual scrolling
• Progressive navigation lets users walk through their own prompts in chronological order
• Make reviewing the task easier.

[github-actions]

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

• [FEATURE]:Can you add a one-click navigation for each round of messages? Scrolling through the messages is too difficult. #23433: Very similar request for one-click navigation between message rounds in the session view
• [FEATURE]: Add keybinds to scroll to prev/next user message #5029: Request for keybinds to scroll to previous/next user message (closed)
• [FEATURE]: Add keybind to navigate between user messages #5077: Request for keybind to navigate between user messages with progressive jump behavior (closed)
• [FEATURE]:Add a go to bottom button for the TUI #11264: Request for a 'Go To Bottom' button in the TUI (closed)
• [FEATURE]: Add 'Jump to Answer Start' Button #9794: Request for 'Jump to Answer Start' button alongside existing navigation (closed)

For keybind-related issues, please also check our pinned keybinds documentation: #4997

[simonklee]

The underlying navigation behavior already exists through the configurable messages_previous / messages_next commands, and End (messages_last) returns to the bottom.
The remaining request is specifically for visible, clickable navigation controls. That substantially overlaps with:

• [FEATURE]:Can you add a one-click navigation for each round of messages? Scrolling through the messages is too difficult. #23433 for one-click navigation between message rounds
• [FEATURE]: Scroll down ui button when !bottom #5783 / [FEATURE]:Add a go to bottom button for the TUI #11264 for a conditional scroll-to-bottom button
• [FEATURE]: Add keybind to navigate between user messages #5077 / Add keybindable commands to navigate between user messages #5078 for the existing user-message navigation behavior
  Closing this as a duplicate of [FEATURE]:Can you add a one-click navigation for each round of messages? Scrolling through the messages is too difficult. #23433.