fix(ams): show "?" not "Empty" for non-RFID spools using tray_exist_bits (#2527)

A spool with no readable RFID was reported by the standard AMS with an empty
tray_type and state=9 — structurally identical to a truly-empty slot at the
tray level — so the AMS card rendered it "Empty" while Bambu Studio correctly
showed "?". The authoritative "a spool is physically here" signal is firmware's
AMS-level tray_exist_bits bitmask (what Studio uses), but Bambuddy inferred
emptiness from the per-tray state/tray_type. Confirmed from the reporter's
bundle: tray_exist_bits=f (all four slots present) with tray_is_bbl_bits=5
(only slots 0,2 Bambu) — the present-but-non-Bambu slots were the ones shown
Empty. Supersedes closed #1838.

apply_tray_exist_bits() already parses the bitmask to clear stale fields on
absent slots; it now also annotates each slot with an authoritative `exists`
bool, gated behind a new annotate_exists flag so only the printer-card path
sets it. The VP bridge leaves it off, so the `exists` key never reaches the
slicer wire format. `exists` flows through the AMSTray schema/serialization to
the frontend, where getEmptySlotKind() uses it: exists===true + no tray_type
-> "?" (present, unconfigured), exists===false -> "Empty", exists absent ->
the previous state=9/10 heuristic (AMS-HT and missing-bitmask paths unchanged).
H2D/X1C already reported present-unknown slots with a non-9 state and took the
"?" path; with the fix they reach it via `exists` and are unaffected.

frontend/src/api/client.ts

@@ -365,6 +365,7 @@ export interface AMSTray {
   drying_temp: number | null;      // RFID-recommended drying temp
   drying_time: number | null;      // RFID-recommended drying time (hours)
   state: number | null;            // AMS tray state: 9=empty, 10=spool present not loaded, 11=loaded
+  exists?: boolean | null;         // Firmware tray_exist_bits: spool physically present (non-RFID → "?" not "Empty", #2527)
 }
 
 export interface AMSUnit {