fix(ui): global tabs + pane stability

[vultuk]

This PR addresses multiple UX regressions:

• Restore global tabs; panes are optional (not forced).
• Do not auto-enable tiled mode on load; respect persisted state.
• Keep global tabs visible in tiled mode (VS Code-like: tabs target active pane).
• Pane "+" opens a session picker (Shift+click opens folder browser).
• Split-pane terminals now replay output buffer on join (render 'session_joined' output).
• Minor CSS cleanup: remove rule that hid tabs in tiled mode.

This resolves:

• Single pane taking half width (already fixed in 3.0.3 overlay change).
• Switching tabs creating the impression of a new session (was showing empty because we didn't replay buffer).
• Closing a tab now deletes the session and removes it from all panes.
• New panes can attach to existing sessions via picker; sessions work immediately.
• Pane layout persists across refresh without forcing tiled mode.

[Copilot]

Pull Request Overview

This PR restores global tabs functionality and improves pane stability in the UI by making multi-pane mode optional rather than forced. The changes address several UX regressions related to tab visibility, session management, and pane behavior.

Key changes:

• Remove forced multi-pane mode activation, making it respect user preferences
• Keep global tabs visible in all modes (single pane and tiled)
• Add session buffer replay when joining existing sessions in split panes

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/public/style.css	Removes CSS rules that forcibly hid global tabs in tiled mode
src/public/panes.js	Adds session buffer replay on join and improves pane "+" button behavior
src/public/app.js	Removes auto-enabling of multi-pane mode to respect user preferences

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[Copilot]

The regex pattern /\x1b\[\[?[IO]/g uses a magic regular expression without explanation. Consider adding a comment explaining what ANSI escape sequences are being filtered, or extract this to a named constant like ANSI_IO_FILTER_REGEX.

[Copilot]

The openAddMenu method is called but not defined in the visible code context. Verify that this method exists in the PaneManager class, as calling an undefined method will result in a runtime error.