fix: usb target picking excludes EFI partitions and pre-flights space

usb_detect offered a Ventoy stick's 32M VTOYEFI ESP as a sync/backup
target: with the data partition unmounted it was the only mounted
candidate, and cp died mid-copy with 'No space left on device'.
Detection now reads FSTYPE/PARTTYPENAME and drops EFI system
partitions from both the mounted list and the mount-offer list;
USB_MOUNTED entries carry mp|label|size|model|fs and usb_pick_root
shows that in the single-stick confirm and the multi-stick/partition
picker (USB_ROOT stays a bare mountpoint). pos-media-sync pre-flights
the exact payload size vs df free space (err, or warn under --dry-run)
before any copy.

DOC/AGENT_Context_Project.md

@@ -574,7 +574,7 @@ Use conventional prefixes: `feat:`, `fix:`, `docs:`, `refactor:`, `chore:`
 | `lib/entertainment-plugin-lib.sh` | 67 | Message-safe helpers for plugins (config load, require, fetch+retry) — plugins MAY source it |
 | `lib/scheduler-lib.sh` | 760 | Scheduler lib (job parsing, notify policies, per-job user timers via user-timers-lib, legacy migrate) |
 | `lib/user-timers-lib.sh` | 112 | Shared systemd **user** timer machinery (interval→OnCalendar, unit pair writer, linger) |
-| `lib/usb-lib.sh` | 194 | Shared USB-storage detection + pick flow (detect/mount-offer/`usb_pick_root`) — used by `pos system backup` + `pos media sync` |
+| `lib/usb-lib.sh` | 205 | Shared USB-storage detection + pick flow (detect/mount-offer/`usb_pick_root`; EFI system partitions excluded; picker shows size/label/fs) — used by `pos system backup` + `pos media sync` |
 | `bin/flag-reader` | 58 | Inspect flags (list/status/`--raw`) |
 | `bin/flag-set` | 21 | Set a flag (optionally with a value) |
 | `bin/flag-clear` | 21 | Unset a flag |
@@ -599,7 +599,7 @@ Use conventional prefixes: `feat:`, `fix:`, `docs:`, `refactor:`, `chore:`
 | `bin/pos-entertainment-status` | 62 | Show enabled plugins and scheduler state |
 | `bin/pos-media-mp3` | 86 | Download audio as MP3 (yt-dlp) |
 | `bin/pos-media-mp4` | 132 | Download video as MP4 (smart/interactive format select) |
-| `bin/pos-media-sync` | 138 | Incremental Music → USB sync (mp3/mp4, add/update only) |
+| `bin/pos-media-sync` | 164 | Incremental Music → USB sync (mp3/mp4, add/update only) |
 | `bin/pos-network-checkport` | 496 | Check TCP/UDP port reachability (nmap, or bash/nc fallback) + local interface view |
 | `bin/pos-network-download` | 951 | aria2 RPC daemon + queue control (add/torrent/metalink, watch, limits) |
 | `bin/pos-network-hotspot` | 93 | Wi-Fi hotspot via create_ap + wihotspot-gui |

DOC/SCRIPTS.md

@@ -210,7 +210,7 @@ Sourced by `bin/pos-entertainment-send|config|enable|disable|status` (after `lib
 ## lib/usb-lib.sh — shared USB-storage detection
 
 **File:** `lib/usb-lib.sh` (installed to `/usr/local/bin/usb-lib.sh`)
-**Purpose:** the one copy of the USB-storage machinery shared by `pos system backup`'s post-verify USB copy and `pos media sync` — `usb_detect` (lsblk JSON, TRAN + lsusb/by-id cross-check → `USB_MOUNTED`/`USB_UNMOUNTED`), `usb_related_present`, `usb_mount_offer` (mount an unmounted stick at `/media/<label>`, `usb-automount` scheme), and `usb_pick_root` (detect → mount-offer → single/multi picker → `USB_ROOT`). Defines only `usb_*`; seams `USB_MOUNT_BASE` (default `/media`, alias `BACKUP_MOUNT_BASE`) and `USB_BYID` (default `/dev/disk/by-id`, alias `BACKUP_USB_BYID`) keep existing config lines working.
+**Purpose:** the one copy of the USB-storage machinery shared by `pos system backup`'s post-verify USB copy and `pos media sync` — `usb_detect` (lsblk JSON, TRAN + lsusb/by-id cross-check → `USB_MOUNTED` as `mp|label|size|model|fs` entries and `USB_UNMOUNTED` as `path|label|size|model`; EFI system partitions — Ventoy `VTOYEFI`, `/boot/efi` — are excluded from both), `usb_related_present`, `usb_mount_offer` (mount an unmounted stick at `/media/<label>`, `usb-automount` scheme), and `usb_pick_root` (detect → mount-offer → single-confirm or multi-picker, rows showing size/label/fs → `USB_ROOT` set to the bare mountpoint). Defines only `usb_*`; seams `USB_MOUNT_BASE` (default `/media`, alias `BACKUP_MOUNT_BASE`) and `USB_BYID` (default `/dev/disk/by-id`, alias `BACKUP_USB_BYID`) keep existing config lines working.
 
 ---
 

DOC/howto/media.md

@@ -116,6 +116,14 @@ The source folder may be a symlink to a library elsewhere
 (`~/Music -> /mnt/data/music`) — it is followed, the artist/album tree is
 mirrored under the symlink's target.
 
+**Target picking.** Mounted USB partitions are listed with size, label and
+filesystem (single candidate → confirm prompt; several → numbered picker, one
+row per partition). EFI system partitions (e.g. a Ventoy stick's `VTOYEFI`,
+32 MB) are **never** offered as a target — they are boot machinery, not
+storage. Before any copy the tool verifies the payload fits (`df` vs the
+exact bytes to copy) and fails fast with `Not enough free space on …` instead
+of dying mid-copy with `No space left on device`.
+
 | Flag | Meaning |
 |------|---------|
 | `--mp3` | Sync only `*.mp3` (neither flag = both) |