Modularize sticky notes and fix code review findings

[salmonumbrella]

Splits the monolithic index.ts into focused modules (constants, layout store, Roam API) and addresses review findings:

• Extract constants, layout persistence, and Roam API helpers into separate files
• Replace N+1 startup queries with single Datalog query
• Batch DOM appends with DocumentFragment
• Defer title font measurement until after DOM insertion
• Drop unused fields from internal API surface
• Document intentional behavior changes

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