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T-15 — Decompose usageDb.js (969→40 lines): - Extract src/lib/usage/migrations.js (legacy + JSON→SQLite migration) - Extract src/lib/usage/usageHistory.js (tracking, pending, log.txt) - Extract src/lib/usage/costCalculator.js (pure cost calculation) - Extract src/lib/usage/usageStats.js (dashboard aggregation) - Extract src/lib/usage/callLogs.js (structured logs, CRUD, rotation) - usageDb.js is now a thin facade re-exporting all functions T-28 — Decompose handleSingleModelChat (183→80 lines): - Extract handleNoCredentials() — credential error responses - Extract safeResolveProxy() — proxy resolution with error handling - Extract safeLogEvents() — fire-and-forget proxy + translation logging - Also created chatHelpers.js with standalone helper exports T-29 — Extract shared UI primitives (3230 total lines): - FilterBar.js — search input + filter chips dropdown - ColumnToggle.js — table column visibility toggle - DataTable.js — generic data table with sticky header, loading/empty Tests: 88/88 pass (no regressions)