fix(server): default data dir to ~/.sideshow with one-time migration

[benvinegar]

Problem

The default data directory was package-relative (<package-root>/data/), which causes two failure modes:

1. EACCES on root-owned globals — sudo npm install -g sideshow puts the package under /usr/lib/node_modules/ (root-owned). Even after fix: create sqlite db parent directory before opening #157's mkdirSync guard (which fixed the ENOENT case for user-owned installs), the mkdir itself throws EACCES against a read-only parent. The server still crashes.

2. Data wiped on reinstall — data under node_modules/sideshow/data/ is deleted by npm install -g on upgrade. A user who runs sideshow for weeks, upgrades, and loses their workspace gets no warning.

Fix

Move the default data directory to ~/.sideshow/ — user-owned, always writable, survives reinstalls.

• SIDESHOW_DATA / SIDESHOW_DB now default to ~/.sideshow/sideshow.{json,db} instead of <pkg>/data/sideshow.{json,db}
• Env overrides are unchanged — a user who set either one is managing their own location
• A one-time file-level migration copies any existing sideshow.{db,db-wal,db-shm,json} from the old <pkg>/data/ to ~/.sideshow/ on first boot (only when using default paths)
• The migration is idempotent (never overwrites files at the new location — safe on every boot), uses renameSync with an EXDEV → cpSync fallback for cross-filesystem moves, and leaves old files in place as a backup
• Composes with the existing migrateJsonToSqlite: a copied JSON file still gets imported into a fresh SQLite db at the new location

Verification

• 6 new unit tests in test/migrateDataDir.test.ts (migrate all files, idempotent, no-op on missing old dir, no-op on no sideshow files, leaves unrelated files, partial new dir)
• Full suite: 333/333 pass
• npm run typecheck, npm run lint, npm run format:check all clean
• End-to-end smoke test: booted with a fresh HOME → migration ran (old data/ → ~/.sideshow/), JSON→SQLite import ran, server started; second boot was idempotent (no migration log)

Files

• server/migrateDataDir.ts — migrateLegacyDataDir(oldDir, newDir): one-time file migration
• server/index.ts — new ~/.sideshow/ defaults + migration call (guarded by !SIDESHOW_DATA && !SIDESHOW_DB)
• test/migrateDataDir.test.ts — 6 tests
• .changeset/sideshow-data-home.md — patch

Closes #160.