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Add a POST /widgets endpoint that validates a name field and inserts into D1.

V0 protocol passed. V1 lint passed. V2 oracle: 9/9 acceptance criteria met. V3 integration passed.
PR opened: add-widgets-endpoint — 3 files changed, tests passing.

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Issue #482: Export button throws on empty selection

Triaged. Fix built. Tested against real suite: 42/42 passing. PR #483 opened.