fix: correct FK direction when dragging from parent to child table

[20syldev]

Summary

When a user drags from a primary key field (the "one"/parent side) to a non-unique field (the "many"/child side), the relationship was stored with the PK table as startTable. Since all SQL exporters generate ALTER TABLE [startTable] ADD FOREIGN KEY(...), this produced incorrect SQL that placed the FK constraint on the parent (referenced) table instead of the child (FK holder) table.

Two complementary fixes:

Fix A — Normalize at creation time (Canvas.jsx): When getCardinality() returns ONE_TO_MANY (start field is unique/PK, end field is not), swap startTable/endTable and change cardinality to MANY_TO_ONE. This ensures startTable always represents the FK holder going forward.

Fix B — Safety net in exporters (shared.js + all SQL exporters): Added resolveFKDirection(r) helper that checks if a relationship has ONE_TO_MANY cardinality and swaps the FK/referenced table assignment accordingly. This handles any pre-existing diagrams saved before this fix.

Before

-- User drags from members.id (PK) → memberships.member_id
-- Wrong: FK placed on the referenced table
ALTER TABLE `members`
ADD FOREIGN KEY(`id`) REFERENCES `memberships`(`member_id`)

After

-- Correct: FK placed on the child table
ALTER TABLE `memberships`
ADD FOREIGN KEY(`member_id`) REFERENCES `members`(`id`)

Files changed

• src/components/EditorCanvas/Canvas.jsx — normalize direction at creation time
• src/utils/exportSQL/shared.js — add resolveFKDirection() helper
• src/utils/exportSQL/mysql.js, mariadb.js, postgres.js, mssql.js, oraclesql.js — use resolveFKDirection()
• src/utils/exportSQL/generic.js — all 5 FK sections updated

Test plan

• Create a members (id PK) and memberships (member_id) table
• Drag from members.id to memberships.member_id to create a 1:N relationship
• Export to SQL — verify ALTER TABLE memberships ADD FOREIGN KEY(member_id) REFERENCES members(id)
• Drag in reverse (from memberships.member_id to members.id) — verify same correct output
• Load an old diagram with a ONE_TO_MANY relationship and verify export is now correct

Fixes #873

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