[FEATURE]: Add stronger visual styling controls for TUI session tabs

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

Session tabs in the V2 TUI can visually blend into the conversation text, making the tab strip and selected tab difficult to identify at a glance. The current tabs.indicators setting switches between status icons and numbers, but numbers do not address the underlying issue: weak structural separation between navigation and chat content.

Please add styling controls that can make session tabs visually distinct without relying only on color. Useful options could include:

• selected and inactive tab borders or separators
• tab padding and spacing
• a distinct tab-strip background
• stronger active-tab emphasis
• a dedicated TUI plugin slot for replacing or decorating the tab strip

This would improve scanability and accessibility for users whose terminals, themes, or visual needs make color-only differentiation insufficient.

The issue is consistently noticeable during normal use. Numbered indicators are not a suitable workaround because the problem is visual separation rather than identifying tabs by number.

Environment:

• OpenCode: opencode2 v0.0.0-beta-18992
• OS: Microsoft Windows 10.0.26200.9168
• Terminal: TERM=xterm-256color; terminal application unavailable
• Shell: C:\WINDOWS\system32\cmd.exe
• Install/channel: beta
• Active plugins: none found in configuration

Related but distinct: #42416 requests a per-session plugin indicator slot, and #38985 requests busy-status indicators. This request is specifically about the visual separation and styling of the tab strip itself.