feat(AppShell): add Back/Forward tree navigation history

[alexwarren]

Summary

• Add Back/Forward toolbar buttons that retrace previously-selected tree elements, similar to browser history — a nice-to-have carried over from the v5 desktop editor. Implemented as a client-side selection-history stack in editor-store.ts (no URL/routing changes), kept in sync across element renames and deletions.
• Remove the "Quest Viva's editor is in preview" yellow banner from the Open/Create screen — the editor is considered stable enough now that this warning is no longer accurate.
• Trim the tree panel's per-row vertical padding (4px → 2px per side) for a slightly more compact view.

Test plan

• npx svelte-check passes with 0 errors
• Manually verified in browser: created a local draft game, selected game → room → player in the tree, confirmed Back/Forward buttons enable/disable correctly and retrace selection in both directions
• Confirmed the yellow preview banner no longer renders on the Open screen
• Confirmed tree rows render with tighter spacing

🤖 Generated with Claude Code