add chunked upload to bypass 100MB proxy body limit

Resumable /api/uploads flow (init -> append -> complete -> abort) so a
large recording uploads as many small slices (<= ASR_CHUNK_SIZE_MB, default
50MB) instead of one request a size-capping proxy (Cloudflare's 100MB) would
reject. The web console auto-routes files larger than one slice through the
chunked path with cross-slice progress and transient-error retry; smaller
files keep the single-shot upload.

- store: in-memory upload sessions + .part reassembly (offset writes,
  out-of-order/idempotent), reuse upload id as task id, orphan sweep on
  start, idle-session GC
- fix limitedReader rejecting bodies exactly at the cap (broke full chunks)
- fix drag-drop double-firing when a drop lands on the dropzone (its drop
  handler now stopPropagation so the window listener doesn't re-upload)
- tests: store + HTTP flow, plus a 120MB reassembly test (ASR_TEST_LARGE)

internal/store/errors.go

@@ -4,3 +4,7 @@ import "errors"
 
 // ErrNotFound is returned when a task id does not exist.
 var ErrNotFound = errors.New("task not found")
+
+// ErrUploadNotFound is returned when a chunked-upload session id does not exist
+// (never created, already completed, or swept after being idle too long).
+var ErrUploadNotFound = errors.New("upload session not found")