[client] Report file drop progress, and stop a withdrawn transfer

The receiver reported progress once per file, after the whole body had
been staged: spool.Write drains its reader before returning, so a large
file sat at nothing until it jumped to done. It now stages through a
reader that reports as the bytes land, the same way the sender already
did, and both sides thin their reports to one per percent and one per
200ms so a fast transfer cannot flood the history or the UI.

Progress also becomes an event of its own. It was left out because a
report per byte would have been unusable; thinned, it costs a handful of
events a second and spares every UI a poll. The daemon's event bridge
ignores the new kind, so nothing is published where a notification would
be noise.

Withdrawing consent mid-transfer did nothing. Cancel routed through
Decide, which only moves an offer out of Pending, so an accepted offer
kept its decision, kept its spool, and kept being uploaded into: the
receiver's list went quiet while the sender ran to completion. Revoke
takes an offer back whatever it has already answered, the staging reader
gives up as soon as consent is gone, and the sender stops rather than
retrying a refusal three times over. A transfer withdrawn this way reads
as declined on both ends, which is what it is, rather than as a failure.

client/internal/filedrop/client.go

@@ -43,13 +43,6 @@ type ClientConfig struct {
 	OfferTimeout time.Duration
 }
 
-type progressReader struct {
-	r      io.Reader
-	sent   int64
-	total  int64
-	report func(sent int64)
-}
-
 // Client sends offers and payloads to a peer's file drop service.
 type Client struct {
 	http         *http.Client
@@ -58,15 +51,6 @@ type Client struct {
 	offerTimeout time.Duration
 }
 
-func (p *progressReader) Read(b []byte) (int, error) {
-	n, err := p.r.Read(b)
-	if n > 0 {
-		p.sent += int64(n)
-		p.report(p.sent)
-	}
-	return n, err
-}
-
 // NewClient builds a sending client over the given dialer.
 func NewClient(cfg ClientConfig) (*Client, error) {
 	if cfg.Dial == nil {
@@ -301,6 +285,12 @@ func (c *Client) uploadFile(ctx context.Context, base string, id OfferID, index
 			if ctx.Err() != nil {
 				return ctx.Err()
 			}
+			// A refusal and a vanished offer are answers, not transport
+			// hiccups: the receiver has withdrawn, and retrying only keeps
+			// pushing bytes at someone who said no.
+			if errors.Is(err, ErrNotAccepted) || errors.Is(err, ErrOfferNotFound) {
+				return err
+			}
 			lastErr = err
 			log.Debugf("upload attempt %d for %s: %v", attempt+1, p.Meta.Name, err)
 			continue
@@ -355,6 +345,9 @@ func (c *Client) putFile(ctx context.Context, base string, id OfferID, index int
 	if resp.StatusCode == http.StatusForbidden {
 		return ErrNotAccepted
 	}
+	if resp.StatusCode == http.StatusNotFound {
+		return ErrOfferNotFound
+	}
 	if resp.StatusCode != http.StatusNoContent && resp.StatusCode != http.StatusOK {
 		return statusError(resp)
 	}