fix(sheetset): wasm build, lock files and sheet set field refresh

- `.dst` read/write go through native-only helpers (a browser answers
  "Not available"); the folder pickers are web no-ops; time stamps use
  iced::time (web-safe).
- Lock files are written only for drawings that are sheets of an open set,
  released when the last set closes, and a lock a crashed session left
  (recorded with its process id, same user and machine) is taken back.
  The `.dwl2` header is well-formed and the date line comes from the OS
  long date on every platform. The 1 Hz poll runs only while a set is open.
- Refreshing sheet set fields never stores `####`: fields no open set
  resolves keep their value, and only the hosts that changed are redrawn
  (and their tab marked modified).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/ui/window/sheet_set.rs

@@ -67,7 +67,7 @@ pub struct SheetSetManager {
     pub seen: std::collections::HashMap<String, std::time::SystemTime>,
     /// Sheet status by sheet id (SSMSHEETSTATUS) and when it was taken.
     pub status: std::collections::HashMap<String, SheetStatus>,
-    pub status_at: Option<std::time::Instant>,
+    pub status_at: Option<iced::time::Instant>,
     /// Sheet Views: by category (else by sheet).
     pub by_category: bool,
     /// Model Views: location id, folder and its drawings.