fix(tui): make session scrollbar visible by default with better contrast

[beiliya66677-wq]

Problem

The right-side scrollbar in the session view is invisible by default on Windows Terminal (and other dark terminals). Two issues:

1. Default hidden: the scrollbar_visible KV signal defaults to false (set by PR fix(tui): restore hidden session scrollbar default #20947), so users must manually toggle it via Ctrl+P → Toggle session scrollbar
2. Invisible colors: when enabled, the scrollbar track uses theme.backgroundElement (#1a1a1a) on theme.background (#0a0a0a) and the thumb uses theme.border (#262626) — both nearly indistinguishable on dark backgrounds

Changes

packages/tui/src/routes/session/index.tsx:

1. Default visibility: changed scrollbar_visible KV default from false → true
2. Track color: backgroundColor from theme.backgroundElement → theme.backgroundPanel for subtle visual distinction
3. Thumb color: foregroundColor from theme.border → theme.textMuted for a visible gray thumb across all themes

Closes #25981

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The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate Found:

• PR feat(tui): add visible white scrollbar to session chat #27795: feat(tui): add visible white scrollbar to session chat (feat(tui): add visible white scrollbar to session chat #27795)

Why it's related: This PR addresses a similar issue of making the session scrollbar visible and improving its appearance. Both PRs focus on improving scrollbar visibility and contrast in the session view. PR #27795 may have been a previous attempt or related work on the same problem.

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