feat: dynamic column mapping for Run AI

[aaronbrezel]

Summary

• Replaces hardcoded column names (source_drive, system_prompt, etc.) and AIMode with a RunConfig interface where users select column mappings at runtime via the sidebar
• Rewrites runBatchAI to accept RunConfig — validates all selected columns by name, auto-creates output column if missing, supports explicit row range or falls back to active sheet selection
• Replaces the TEXT/FILE modal dialog with an inline sidebar config panel: reads sheet headers on open, presents checkbox lists for user prompt + Drive file columns, dropdowns for system prompt + output column, and a row range toggle

Changes

• src/shared/types.ts — removes AIMode, ColumnMap, ColumnConfig; adds RunConfig
• src/server/config.ts — removes COLUMNS from CONFIG
• src/server/inference.ts — driveLinks and systemPrompt are now optional params with explicit !== undefined guards
• src/server/utils.ts — adds resolveColumns(headers, names): number[] pure helper
• src/server/index.ts — rewrites runBatchAI(config: RunConfig), adds getSheetHeaders(), removes showSourceDialog/handleDialogSelection
• src/server/dialog.ts — deleted
• rollup.config.js — footer stubs updated to match new exports
• src/Sidebar.html — inline AI config panel replaces old modal trigger

Test Plan

• 91 tests passing (npm test)
• Coverage thresholds met (npm run test:coverage)
• Zero typecheck errors (npm run typecheck)
• Zero lint errors (npm run lint)
• Clean build (npm run build)
• Manual: open sidebar → Run AI Inference → verify headers populate
• Manual: select user prompt columns, output column → run on selected rows → verify output written
• Manual: select Drive file columns → verify multimodal path works
• Manual: "New column..." path → verify new column created with ai_ prefix

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