feat(editor): consolidate left panels into one tabbed Explorer panel

[DavidBabinec]

What

Collapses the separate left-rail navigation panels (Layers, Site, Media) into a single Explorer rail button backed by a tabbed ExplorerPanel — Layers / Site / Code / Media — mirroring the earlier Framework-panel consolidation.

Panel

• New ExplorerPanel owns the shell (header + SegmentedControl tabs + close); each tab renders its panel in a headerless tab variant (new Panel headerless prop).
• DomPanel is now tab-only — removed the dead floating/docked machinery (useDraggablePanel, PanelHeader, collapse toggle, width persistence) and the backing domTreePanel store state, setDomTreePanel/toggleDomTreePanel actions, and the instatic-dom-panel storage key.
• SiteExplorerPanel / MediaExplorerPanel render chrome-free as tab bodies.

Tabs

• Layers → DomPanel. The search row gains an Insert module button reusing the shared ModuleInserterDialog + useInsertInserterItem (identical target resolution to the canvas + / toolbar + Add).
• Site → SiteExplorerPanel sectionGroup="site" — Pages, Templates, Components. "New component" now uses the same + icon as New page / New template.
• Code → the same SiteExplorerPanel instance with sectionGroup="code" — Styles + Scripts. One shared mount so Site/Code share one DnD scope + selection (two instances would each register useDndMonitor and double-handle every drag).
• Media → MediaExplorerPanel — drops the "Other" bucket (images/videos only), defaults to grid view, shows EXT · size per item instead of the URL, 16:9 tile preview, softer --bg-surface-2 tile hover.

Rail / store

• One explorer rail item (DatabaseSolidIcon, gold accent — both carried over from the standalone Layers button) instead of three. explorerPanelOpen + explorerPanelTab replace the per-panel open flags.

Misc

• Code editor floats above the docked sidebars (z-index 80→95) so it no longer hides behind the Explorer panel.
• Spotlight panel commands open the Explorer panel on the matching tab.
• Removed the unused workspace/pagesPanel plumbing that anticipated Content-page reuse (Content has its own ContentSidebar).

Why

Three near-identical navigation panels each consumed a rail slot; consolidating them into one tabbed panel matches the Framework-panel pattern, frees rail space, and lets the always-reserved chrome live in one shell. Splitting source files (Styles/Scripts) out of the mislabeled "Pages" tab into a dedicated Code tab clarifies the taxonomy (renderable structure vs. raw source).

Impact

• User-facing: one Explorer button with Layers/Site/Code/Media tabs; default Layers; active tab persists per-workspace.
• Pre-release, no backward-compat: a previously-stored explorerPanelTab: 'pages' falls back to the default (layers).

Verification

• bun run build ✓
• bun run lint ✓
• bun test → 5817 pass; the single failure (OnboardingPanel framework import) is pre-existing on main, unrelated to this change.
• Live browser check: 4 tabs render; Site/Code section split; code editor floats over the panel; rail accent unchanged.

🤖 Generated with Claude Code