fix: improve CalDAV task detection for external clients (Thunderbird) #84

Two issues prevented tasks created in Thunderbird (or other CalDAV
clients) from appearing in the task view:

1. percentComplete was not in CALENDAR_TASK_PROPERTIES, so it was
   never requested from the server and the heuristic check for it
   was always false (dead code).

2. The hasTaskFields heuristic used strict value checks:
   - 'progress' in obj && typeof obj.progress === 'string'
     → fails when Stalwart returns progress: null instead of the
       RFC 8984 default "needs-action"
   - 'due' in obj && obj.due != null
     → fails when Stalwart includes due: null for tasks without a
       DUE date (key present, value null)

RFC 8984 §5.2 defines due, progress and percentComplete as Task-only
properties — a VEVENT will never include them in a JMAP response.
Checking for key presence alone (even when null) is therefore a
reliable discriminator, regardless of the actual value.

lib/jmap/client.ts

@@ -189,6 +189,7 @@ const CALENDAR_TASK_PROPERTIES = [
   'useDefaultAlerts',
   'alerts',
   'relatedTo',
+  'percentComplete',  // Task-only per RFC 8984 §5.2.4 — used in detection heuristic
 ] as const;
 
 /**
@@ -3974,9 +3975,13 @@ export class JMAPClient implements IJMAPClient {
       const isExplicitTask = typeof type === 'string' && type.toLowerCase() === 'task';
       // CalDAV-created tasks (e.g. Thunderbird) may lack @type or have @type
       // set to something other than 'Event'. Detect them by the presence of
-      // task-specific fields: progress, due, or percentComplete.
-      const hasTaskFields = ('progress' in obj && typeof obj.progress === 'string')
-        || ('due' in obj && obj.due != null)
+      // task-specific keys (due, progress, percentComplete), which RFC 8984 §5.2
+      // defines as Task-only — a VEVENT will never include them in the response.
+      // We check for key presence (even if null) because Stalwart may return null
+      // instead of the RFC defaults (e.g. progress default is "needs-action").
+      // @see https://www.rfc-editor.org/rfc/rfc8984#section-5.2
+      const hasTaskFields = ('due' in obj)
+        || ('progress' in obj)
         || ('percentComplete' in obj);
       const isCalDavTask = type !== 'Event' && hasTaskFields;