feat(viewer): add per-panel edge docking (top/right/bottom/left)

[ericgozzi]

Toolbar, Openbar, Metadata, and ObjectActions can each independently dock to any of the viewer's four edges via toolbarPlacement/openbarPlacement/ metadataPlacement/objectActionsPlacement options (default: toolbar+openbar left, metadata+objectActions right - unchanged from today's floating-corner look).

• New generic PanelDock.vue: a positioning zone for one edge, with no knowledge of which panels land in it. Toolbar and Openbar are not wrapped in a shared component - App.vue mounts all four panels independently, grouped only by matching placement, in a fixed priority order per edge (toolbar, openbar, metadata, objectActions).
• Deletes Sidebar.vue/RightSidebar.vue. Fixes a pre-existing bug where ObjectInfo was mounted twice (once directly in App.vue, once inside RightSidebar).
• Renames ObjectInfo.vue -> Metadata.vue, now self-contained: owns its own show/ hide toggle and edge-aware collapse (previously split between ObjectInfo's close button and RightSidebar's open button).
• .app-container uses a 3x3 CSS Grid border layout (top/bottom docks span the full width; left/right docks are confined to the row between them) instead of four independent position: absolute overlays, so a top dock and a left dock no longer overlap at the shared corner.
• Panels sharing the top/bottom edge stack as separate full-width bars (column layout), not packed side by side into one row.
• Each panel adapts row vs. column internal layout and flips its collapse-arrow direction based on its own assigned edge.

Adds examples/embedded_panel_docking.html: an interactive demo with a dropdown per panel to flip placements live, plus a send handler that simulates a backend responding to object selection (object_infos/object_action), so Metadata and Object actions populate when you click the box - the same round trip a real backend integration would go through.

Verified with vue-tsc, eslint, the full vitest suite (41 tests, incl. new tests/panel_docking.test.ts), and headless-browser checks against the built dist-lib/ output confirming no overlap and correct stacking order.

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Checklist

• npm run check is green (formatting, lint, types, unit tests, both builds).
• npm run test:browser and npm run test:package pass.