Isolate DeveloperChat and DashboardChat sidebar state

[ericpgreen2]

Polishing the in-flight cloud editing feature on two fronts:

1. /-/edit/... should have a single AI surface — the developer agent. Today, viewing a dashboard preview inside the editor mounts both <DeveloperChat /> (root edit layout) and <DashboardChat /> (the /-/edit/explore/[name] wrapper layout), gated on the same chatOpen flag. With both feature flags enabled the user sees two AI panels side-by-side. Removing the dashboard chat in editor routes also moves us toward the future where one Rill Developer AI surface handles both developer and analyst work.
2. Developer and dashboard chat sidebars should have fully independent state. Open/closed state and conversation ID are now scoped per surface. The motivation is the Publish flow: clicking Publish opens a new prod tab, and Chromium clones sessionStorage to that new tab. Without per-surface scoping, chat-open=true carries over (so the prod sidebar appears open by default) and the dev-runtime conversation ID is loaded against the prod runtime, leading to an "Unable to find this conversation" error.

Implementation:

• Delete /-/edit/explore/[name]/+layout.svelte, the wrapper that mounted <DashboardChat /> next to the editor's <DeveloperChat />. Suppress the dashboard <ChatToggle /> in ProjectHeader.svelte when editContext is true.
• Split chatOpen/sidebarActions into per-surface developerChatOpen/developerChatActions and dashboardChatOpen/dashboardChatActions. SidebarChat and ChatToggle now accept the surface's open/actions as props; non-component callers (ExplainAndFixErrorButton, chart-ai-agent, generateCanvas, generate-sample-data, measure-selection, embed public API) wire to their explicit surface.
• Scope BrowserStorageConversationSelector storage key by surface: sidebar-conversation-id-{surface}-{org}-{project}.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

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Developed in collaboration with Claude Code