feat: centralized resource registry

[zmuhls]

Summary

• Add templates.ts store and resources.ts unified aggregator for consistent fetch-once resource management
• Extend themes.ts with createTheme() / deleteTheme() CRUD helpers, eliminating inline fetch+store-update duplication in CanvasToolbar
• Rewire 5 consumers (TemplatesTab, ArtifactsTab, ChatInput, mutations.ts, CanvasToolbar) to use centralized stores instead of 9 scattered fetch calls
• Fix 18 pre-existing type errors across 6 files (ZoneDrop, SlideCard, RichTextEditor, ChatPanel, mutations, debug page) — svelte-check now reports 0 errors
• Rename resource panel label 'Tmpl' → 'Templates'

Test plan

• Templates tab loads and apply-to-slide works
• Artifacts tab loads and insert-into-slide works
• Theme picker: create custom theme, delete custom theme
• Chat @ mentions show both artifacts and templates in autocomplete
• AI applyTemplate mutation + undo/redo still works
• npx vitest run — 613 tests pass
• npx svelte-check --threshold error — 0 errors