feat(library): add Entity-Relationship diagrams (Chen and crow's-foot notations)

[FelixTJDietrich]

Summary

Adds Entity-Relationship diagram support to Apollon, covering the two notations people actually use — shipped as two separate diagram types because they don't share a model:

• Entity-Relationship (Chen) — the academic notation taught in DB courses: entities (rectangles, double for weak), relationships (diamonds, double for identifying), and attributes as ellipses (key = underline, partial-key = dashed underline, multivalued = double ellipse, derived = dashed ellipse). Connectors carry a cardinality label with presets for both the Chen ratio (1/N/M) and (min,max), plus a participation toggle (total = double line).
• Entity-Relationship (Crow's Foot) — the Mermaid / Information-Engineering notation every modern DB tool uses (dbdiagram, DrawSQL, Mermaid erDiagram): entities are structured tables whose columns each carry a name, data type and key role — a PK/FK/UK key gutter, the column name, and an aligned data-type column, with subtle row dividers. Each column is individually themeable (fill/text colour), and all colours flow through the same getCustomColorsFromData path as the class diagram, so theming behaves identically across diagram types. They are related by straight lines with crow's-foot cardinality markers at each end (zero/one/many) and solid (identifying) / dashed (non-identifying) lines. The column layout follows Mermaid's erDiagram / DBML and the classic ER convention of underlining the primary key.

Context: this started from issue #751 (Chen notation). The natural follow-up question was "why doesn't this look like Mermaid's ERD?" — because Mermaid uses crow's-foot, a genuinely different notation (relationships are line-markers, not diamonds; attributes are table rows, not ellipses). Rather than force one model to serve both, Apollon now offers both as first-class types.

Release note

You can now create Entity-Relationship diagrams in both Chen notation (entities, relationships, attributes with key/multivalued/derived decorations) and crow's-foot / Mermaid-style notation (entity tables with crow's-foot cardinality markers).

Implementation notes

• Follows the existing per-diagram pattern (PetriNet/Flowchart/Sfc): each type registers its node/edge types through the shared registries, palette, popovers and generated JSON schema. No new framework.
• Chen: three node types (erEntity/erRelationship/erAttribute) + two edge types; a small pure classifyErConnection gates structurally valid connections and auto-picks connector vs. link.
• Crow's foot: one entity node (erCfEntity) whose columns are a structured ErCfColumn { name, type?, keys?: (PK|FK|UK)[] } model, rendered by a 3-cell row renderer (ErCfRowSection: key gutter | name | type, with the data type aligned in its own column and row dividers drawn from the entity's themeable stroke colour) and edited by a structured column editor (ErCfColumnList) that also exposes a per-column fill/text colour control mirroring the class attribute editor and edited by a dedicated structured column editor (ErCfColumnList: name + type inputs, PK/FK/UK selector, drag-to-reorder). The relationship edge (ErCfRelationship) draws composite end markers via ErCrowsFootMarker (min = bar/ring, max = bar/foot, ordered outward per the IE standard); crow's-foot edges are straight-only so the markers stay attached, and Swap-Ends carries the directional cardinalities.
• Export-safe SVG throughout (plain line/circle/polygon/rect, var(--apollon-background) for open shapes), so PNG/PDF export via resvg renders correctly.

Steps for testing

1. pnpm dev → playground → pick Entity-Relationship (Chen) or Entity-Relationship (Crow's Foot) from the dropdown (or New diagram → Structural).
2. Chen: drag Entity / Weak Entity / Relationship / Identifying / Attribute / Key Attribute; connect entity→relationship → cardinality connector; set a ratio then a (min,max) preset; toggle participation (Total = double line). Connect attribute→entity → link; entity→entity is rejected.
3. Crow's foot: drag Entity tables, edit their columns, connect two entities → a relationship; open its popover and set source/target cardinality + the identifying toggle (solid ↔ dashed); confirm the crow's-foot markers update.
4. Export SVG/PNG/PDF and confirm shapes/markers/labels render.
5. Automated: cd library && pnpm test (Chen + crow's-foot unit tests; full suite green) and the Playwright e2e/visual specs.

Visual baselines for the new ER snapshots are generated in the pinned Playwright container (mcr.microsoft.com/playwright:v1.59.1-noble) via Update Visual Baselines, so they match CI exactly.

Two unrelated infra fixes are included because they fix pre-existing breakage on main that blocks every PR's CI (droppable if fixed independently): style: formats two files left unformatted by #645; ci: fixes the Update-Visual-Baselines workflow's commit step (it set -o pipefail under dash, so it could regenerate baselines but never commit them).

Screenshots / screencasts

Entity-Relationship (Chen) — full notation, weak entity + identifying relationship, key/multivalued/derived attributes:

Entity-Relationship (Crow's Foot) — structured tables (key gutter, name, aligned types, themeable per column), solid/dashed relationships, crow's-foot markers:

Structured column editor (name + type + PK/FK/UK + per-column colour):

Editing cardinality — Chen connector (ratio / min,max presets) and crow's-foot relationship (per-end cardinality + identifying toggle):

Checklist

• Linked to a related issue (if applicable) — closes Entity-Relationship (Chen) notation as a render flavor for class diagrams #751
• Added a changeset whose summary is written in the user's voice
• PR title's Conventional Commit type (feat) matches the kind of change
• Tests added or updated — Chen + crow's-foot unit tests, e2e spec, visual + SVG-export coverage
• Documentation updated (if applicable) — docs/user/overview.md
• Screenshots or screencasts attached