fix(tui): make vertical tabs responsive

[kitlangton]

What

Make the vertical session tab rail the same 42-column width as the session sidebar, and automatically switch configured vertical tabs to the horizontal top strip when the terminal is narrower than 86 columns.

Before / After

Before: the vertical rail varied between 20 and 30 columns. It remained vertical on narrow terminals, squeezing the main session view.

After: both sidebars share one 42-column width constant. Vertical tabs remain enabled when at least 44 columns are left for content; below that breakpoint the existing horizontal strip renders instead.

How

• packages/tui/src/ui/layout.ts owns the shared sidebar width and responsive fit decision.
• The app shell selects the effective tab orientation from terminal width.
• The session route uses the same decision when calculating transcript and right-sidebar space.
• The vertical tab component and session sidebar consume the shared width.

Scope

This does not add a user-configurable sidebar width. tabs.vertical remains the orientation preference, with responsive fallback applied when needed.

Testing

• bun run test from packages/tui: 586 passed, 5 skipped
• bun typecheck from packages/tui
• Push hook workspace typecheck: 33 packages passed
• Real PTY at 120x32: rendered the 42-column vertical rail
• Resized the same PTY to 85x32: rendered the horizontal top strip after redraw

Flow

flowchart LR
  A[Vertical tabs configured] --> B{Terminal width >= 86}
  B -->|yes| C[42-column left rail]
  B -->|no| D[Horizontal top strip]

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