v0.51.18 — 5-PR batch (4 contributor + 1 self-built UX polish)

🐛 Fixed

• #1783 by @Sanjays2402 — Custom provider + :free/:beta/:thinking suffix mis-resolution. Closes #1776 (the v0.51.15 follow-up against PR #1762). @custom:my-key:some-model:free now correctly resolves to provider=custom:my-key, model=some-model:free.
• #1791 by @Michaelyklam — Keep assistant-only stream deltas on current turn. Closes #1787. No more spurious new-turn promotion when only assistant content streams.
• #1790 by @Michaelyklam — Keep workspace open from preview breadcrumb. Closes #1785. New clearPreview({keepPanelOpen:true}) option preserves panel state instead of closing on every preview-clear.
• #1789 by @Michaelyklam — Preserve sidebar scrolling while streaming. Closes #1784. 350ms scroll-intent window suppresses auto-scroll-to-bottom during user gestures (passive listeners, monotonic clock).

✨ Added (UX polish)

• #1794 by @nesquena-hermes — Self-built UX bundle (APPROVED by @nesquena), four fixes:
  • Rail tooltip cascade fix (removed conflicting content:none rule)
  • +new conversation button no longer clipped — new .has-tooltip--bottom-right variant
  • Context-menu hover affordance (typo fix: var(--hover) → var(--hover-bg))
  • Rename modal pre-fills the basename via setSelectionRange(0, dot)

🧪 Tests

4723 → 4747 collected (+24). 4733 passed, 0 failed.

✅ Pre-release verification

• All 5 PRs CI-green individually
• File overlaps (static/style.css + static/ui.js between #1789 + #1794) auto-merged cleanly
• All JS/Python files syntax-clean
• Browser API sanity (11/11 endpoints): all pass
• Opus advisor: SHIP all 5, 0 MUST-FIX, 1 informational SHOULD-NOTE on test pattern divergence (acceptable)

🙏 Closes

• #1776 (custom provider + suffix mis-resolution — was filed in v0.51.15 sweep)
• #1784 (sidebar scroll stolen during streaming)
• #1785 (workspace panel closes on breadcrumb-click from preview)
• #1787 (assistant-only stream deltas creating spurious new turns)

Full Changelog: v0.51.17...v0.51.18