Collapse styles cascade to a single user layer

[kmcginnes]

Description

Collapses the two-layer styling architecture (shared + user) into a single user styles layer. The shared layer was internal and never shipped, so this removes it wholesale with no data migration.

Core change:

• Delete sharedVertexStylesAtom / sharedEdgeStylesAtom and their IndexedDB keys.
• Simplify the cascade to {...appDefault, ...user} — resolveVertexStyle / resolveEdgeStyle no longer take a shared parameter.
• Point styling import/export and the Settings → Styles load flow at the user atoms.

Settings → Styles stays as the home for save/load/reset, now single-layer:

• Three reset buttons (Reset Your / Shared / All Styles) collapse into one Reset.
• Save/Load/Reset get labels and icons matching the config Save/Load in General Settings.

Supporting cleanup:

• Glossary, styling ADRs, and docs/features/settings.md updated to the single-layer model; stale shared-vertex-styles references removed from two adjacent ADRs.
• UI copy says "default style" (not "app default") and uses query-language type labels (Node Label / Class, Edge Label / Predicate).

Out of scope (deferred): the selective import modal (#1972), relocating the buttons to the sidebar, and the styling-export envelope-kind rename.

How to read

1. graphStyles.ts — the cascade collapse; start here
2. useStylingImportExport.ts — import/export repointed to the user atoms
3. storageAtoms.ts — shared atoms deleted
4. ResetStylesButton.tsx — three reset buttons become one
5. SettingsStyles.tsx — single-layer Settings page

Validation

• pnpm checks passes (types, lint, format).
• pnpm test passes.
• Removed tests exercised two-layer-only behavior; single-layer equivalents remain. DbState shared-style helpers removed.

Related Issues

• Closes Collapse to single style layer and retire shared-styles terminology #1974

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have verified pnpm checks passes with no errors.
• I have verified pnpm test passes with no failures.
• I have covered new added functionality with unit tests if necessary.
• I have updated documentation if necessary.