feat(dashboard): replace free-text key inputs with dropdowns in PM integration form

[aaight]

Summary

• Add ConstrainedKeyValueEditor component with a <select> dropdown for the key column, replacing free-text key inputs for known CASCADE PM integration keys
• Define 4 key constant sets: TRELLO_LIST_KEYS (9 keys), TRELLO_LABEL_KEYS (4 keys), JIRA_STATUS_KEYS (7 keys), JIRA_LABEL_KEYS (4 keys)
• Replace Trello Lists, Trello Labels, JIRA Status Mappings, and JIRA Labels KeyValueEditor usages with ConstrainedKeyValueEditor; JIRA Issue Types stays as free-text

Details

• Key column: Native <select> matching existing selects in the file; already-used keys are filtered from other rows
• Add button: Disabled when all allowed keys are used
• Unknown/custom keys: Existing configs with keys not in the allowed set are preserved and shown with (custom) suffix — no data loss
• Single file change: web/src/components/projects/integration-form.tsx only — no backend changes

Test plan

• Trello Lists dropdown shows 9 options; used keys filtered from other rows; Add disabled when all used; Save works
• Trello Labels dropdown shows 4 options; same behavior
• JIRA Status Mappings dropdown shows 7 options; JIRA Labels shows 4 options
• JIRA Issue Types remains free-text KeyValueEditor
• Existing project configs with known keys load correctly into constrained editors
• Existing configs with unknown keys show them with (custom) label and are preserved on save

Card: https://trello.com/c/IhnqywCu/86-status-mappings-and-labels-in-dashboard-in-projects-integrations-configuration-for-project-management-tools-should-feature-the-i

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[nhopeatall]

🔍 Reviewing PR...

[nhopeatall]

LGTM — clean, well-scoped change. Key constants match backend usage exactly (verified against src/pm/trello/integration.ts, src/pm/jira/integration.ts, and src/config/schema.ts). Custom-key preservation ensures no data loss for existing configs. Duplicate-key prevention via otherUsedKeys filtering is correct. The disabled-add-button-when-all-used logic is sound.