feat: add C-a s to toggle the sidebar in boo ui

[kylecarbs]

Closes #47.

The sidebar in boo ui was always visible. C-a s (and C-a C-s) now hides it, giving the viewport the full terminal width, and pressing it again brings the sidebar back at its old width.

Behavior

• C-a s toggles the sidebar; the focused session's pty resizes with the viewport, exactly like a sidebar resize does.
• The hidden state survives terminal resizes (SIGWINCH), like the width preference.
• Mouse hit testing follows: while hidden, every cell belongs to the viewport (wheel, selection, and mouse forwarding work full-width).
• Sidebar-targeting arrows reveal a hidden sidebar before acting, so their effect is never invisible: C-a Up/Down (browse) and C-a Left/Right (resize). Hiding cancels an active browse for the same reason.
• Status content (prompts, messages, the keybind bar) still overlays the bottom row while hidden, since it renders full-width independent of the sidebar.
• The keybind bar, boo help ui, and the README mention the new bind.

Testing

• Unit tests: hidden-layout geometry and hit testing, toggle/restore with browse cancellation, and arrow-reveal behavior.
• Integration test on a real PTY: hides, verifies the session list/hint/separator leave the rendered screen and the session pty grows to full width, types into the hidden-sidebar session, then restores and verifies the sidebar returns at its old width. Added a whole-screen waitScreen helper alongside waitLastRow.
• The keybind-bar test terminal widens from 132 to 140 columns: the bar gained s sidebar and the test asserts its full text renders. The bar remains clipped gracefully on narrower terminals (appendClipped), as before.
• zig build test-all: 136/136 pass; zig fmt --check clean.

Implementation notes

• The hidden flag lives on Layout, next to the geometry it changes (viewportCols, viewportX, hit), so renderer and mouse code need no special cases beyond skipping the sidebar/separator segments in composeRow.
• sidebar_w is preserved while hidden, so show restores the previous width, including a width set by C-a Left/Right.
• The common "viewport geometry changed" tail (live-view resize, selection drop, full repaint) was shared by relayout and applySidebarWidth; it is now factored into viewportChanged() and reused by the toggle.

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This PR was generated by Coder Agents on behalf of @kylecarbs.