chore: misc UX polish across sidebar, observer and onboarding

[rado0x54]

Summary

A bundle of small UX improvements collected from user feedback:

• README wordmark no longer clipped. shellwatch_wordmark-{light,dark}.svg viewBox widened (720 → 820) with matching textLength, so the trailing "H" survives renderers that ignore textLength or substitute a wider fallback font.
• Observer Mode empty-state explainer. When no sessions are open, the grid shows a short panel explaining what Observer Mode is and how sessions appear. Visible to all accounts (the explainer is the gate; no role check).
• Ready-to-copy sshd_config one-liner in both the README's Requirements section and onboarding step 1, so initial setup doesn't require re-typing the config line.
• Sidebar footer redesigned. Account identicon sits on the left, Docs (lucide book-open-text) and GitHub (lucide github) icons on the right. Hovering or clicking the identicon expands the row inline (hiding Docs/GitHub) to show the account name, admin badge, and a Sign-out icon button — same content as the previous always-visible account-info card, just collapsed by default.
  • Hover-expand: collapses 500 ms after the cursor leaves the row; re-entering anywhere in the row cancels the pending collapse.
  • Click-expand: auto-collapses after 3 s regardless of cursor position, so mobile users have time to tap Sign-out.
  • Escape closes the menu, but the keydown listener is only wired while the menu is open — Escape inside an xterm session can't surprise-close it.

Test plan

• Open README.md in the GitHub web UI and in the GitHub mobile app — wordmark renders without clipping
• Visit /observer with no active sessions — empty-state panel renders centered with both paragraphs
• During onboarding, click "Copy" on step 1 → paste in shell → line appended to /etc/ssh/sshd_config (after sudo prompt)
• Sidebar footer: hover identicon → row expands; cursor away → row collapses ~500 ms later
• Sidebar footer: cursor briefly leaves and returns within 500 ms → row stays expanded
• Sidebar footer: click identicon → expands and stays open for 3 s with cursor anywhere
• Sidebar footer: click identicon while click-expanded → collapses immediately
• Click Sign-out in expanded row → logged out and redirected to login
• Press Escape inside an xterm session while sidebar menu is closed — nothing happens
• Press Escape while account menu is open — menu collapses
• Resize to mobile width — hamburger and existing drawer behavior still works