Adds a paused state to the Electron download manager with a full partial-file lifecycle:
- Pause keeps the .part and byte progress; cancel discards them; every lifecycle stage (queued, active, mid-transfer) is pausable.
- Resume continues via HTTP Range with If-Range entity validation (strong ETag / Last-Modified persisted in the new resume_validator column, idempotent migration incl. legacy-table rebuild). Non-206 answers restart from zero over the same .part; the 206 Content-Range offset is verified; responses that end before the advertised size are retained for a Range retry instead of being committed as completed.
- Crash recovery converts interrupted transfers to paused, keeps queued-with-partial rows resumable, and commits finalizations that crashed before the DB update.
- Destination collisions are non-destructive (retained partials finalize to the next numbered name); locked .part files never lose their DB owner across cancel/remove/restart; resume claims rows atomically and the queue dedupes ids.
- Stored request headers are re-filtered through the User-Agent/Origin/Referer allowlist on read, URL-derived extensions are sanitized, resume appends never follow symlinks, and transfer errors are logged by message only.
- UI: pause/resume/cancel/retry/remove surface failures in a snackbar; paused items show an active Resume button in VOD/episode detail views; translations for all 18 locales.
- Runtime split into download-runtime/transfer/finalize/broadcast modules; +30 unit tests and an Electron E2E covering pause -> retained .part -> Range/If-Range resume -> byte-exact assembly.
Co-authored-by: genrichh93-ui <genrichh93@users.noreply.github.com>
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