feat(core,fuse,gui,cli): upload and download bandwidth limits (phase 6)

Every transfer charges the bytes it moves to its direction's rate limit,
a virtual clock shared by all transfers with a one-second burst, and sleeps
off any overshoot. On runtime workers the sleep goes through block_in_place
so FUSE reads keep their threads.

The caps are set live with SetBandwidthLimits, persisted to config and
applied at mount start. Preferences gains a Network group with debounced
MiB/s editors, and the CLI gains `pdfs sync limit --up/--down`.

crates/pdfs-core/src/control.rs

@@ -299,6 +299,10 @@ pub enum Request {
     /// persist it to config so the next mount keeps it. Replies with
     /// [`Response::Ok`].
     SetCacheBudget { bytes: u64 },
+    /// Cap uploads and downloads at these bytes per second (`0` = no cap),
+    /// live for running transfers, and persist both to config so the next
+    /// mount keeps them. Replies with [`Response::Ok`].
+    SetBandwidthLimits { upload: u64, download: u64 },
     /// Report on the health of the metadata database and content cache: sizes,
     /// row counts, and — when `deep` — SQLite's own integrity check. Replies
     /// with [`Response::CacheReport`].
@@ -2419,6 +2423,10 @@ mod tests {
                 id: 3,
                 paused: true,
             },
+            Request::SetBandwidthLimits {
+                upload: 1_000_000,
+                download: 0,
+            },
             Request::SyncNow { id: Some(3) },
             Request::AdoptDevice {
                 uid: Some("dev-1".into()),