Update meetup content for September 2026 Practi"FALL" Prints challenge

- Archive August 2026 Gridfinity Extravaganza
- Set September 2026 Practi"FALL" Prints as active challenge
- Set October 2026 as upcoming challenge
- Create announcements/September2026.md and challenges/September-2026/
- Update Important-Dates, Readme, challenges.md, and sync wiki-content-gitea
- Update wiki-fargo3d submodule pointer

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

wiki-content-gitea/Upcoming-Challenge.md

@@ -6,11 +6,11 @@ tags:
 ---
 [← Back to Home Page](Home)
 
-# Upcoming Challenge: September 2026 (To Be Determined)
+# Upcoming Challenge: October 2026 (To Be Determined)
 
 ## Challenge: [Theme TBD]
 
-*Details for the September 2026 challenge are currently being decided. We'll announce the full details at the August 19th meetup!*
+*Details for the October 2026 challenge are currently being decided. We'll announce the full details at the September 16th meetup!*
 
 ### The Focus: [TBD]
 
@@ -18,7 +18,7 @@ tags:
 
 ### Submission Requirements
 
-* **Physical Model**: Bring your print to the **September** meetup.
+* **Physical Model**: Bring your print to the **October** meetup.
 * **The Story**: Be ready to tell the group about your creation.
 
 ### Judging & Prize
@@ -28,10 +28,10 @@ tags:
 
 ### Timeline
 
-* **Challenge Announcement**: August 19th, 2026 (at August meetup)
-* **Show & Tell / Voting**: September 2026 (during September meetup)
-* **Winner Announcement**: September 2026 (end of September meetup)
+* **Challenge Announcement**: September 16th, 2026 (at September meetup)
+* **Show & Tell / Voting**: October 2026 (during October meetup)
+* **Winner Announcement**: October 2026 (end of October meetup)
 
 ---
 
-**This challenge will become active in September 2026**
+**This challenge will become active in October 2026**