Remove the entire migration module (435 lines) that was used for
one-time migration from v3.1.0 to v3.2.0. This cleans up technical
debt and improves startup performance.
Changes:
- Delete src-tauri/src/migration.rs (copy file scanning and merging)
- Remove migration module reference from lib.rs
- Simplify startup logic to only ensure config structure exists
- Remove automatic deduplication and archiving logic
BREAKING CHANGE: Users upgrading from v3.1.0 must first upgrade to
v3.2.x to automatically migrate their configurations.
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