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/Important-Dates.md

@@ -6,14 +6,14 @@
 
 ### Next Meetup
 
-- **July Meetup: July 15th, 2026** ✓ *Completed*
-  - Judging of July 2026 Challenge ("Scarf Seams")
-  - Challenge announcement for August 2026 Challenge ("Gridfinity Extravaganza")
-
-- **August Meetup: August 19th, 2026**
+- **August Meetup: August 19th, 2026** ✓ *Completed*
   - Judging of August 2026 Challenge ("Gridfinity Extravaganza")
-  - Challenge announcement for September 2026 Challenge ("[Theme TBD]")
-  - Focus on Gridfinity system and organizational efficiency
+  - Challenge announcement for September 2026 Challenge ("Practi\"FALL\" Prints")
+
+- **September Meetup: September 16th, 2026**
+  - Judging of September 2026 Challenge ("Practi\"FALL\" Prints")
+  - Challenge announcement for October 2026 Challenge ("[Theme TBD]")
+  - Focus on practical, functional prints and problem solving
 
 ## Regular Meeting Schedule
 
@@ -61,6 +61,14 @@ See [Challenge Archive](Challenge-Archive) for complete details.
 - **Submission Deadline**: August 19, 2026
 - **Judging**: August 19, 2026 meetup
 
+See [Challenge Archive](Challenge-Archive) for complete details.
+
+### September 2026: "Practi\"FALL\" Prints"
+
+- **Challenge Announcement**: August 19, 2026
+- **Submission Deadline**: September 16, 2026
+- **Judging**: September 16, 2026 meetup
+
 See [Current Challenge](Current-Challenge) for complete details.
 
 ## Meeting Location
@@ -78,4 +86,4 @@ West Fargo, ND 58078
 
 ---
 
-**Last Updated**: August 16, 2026
+**Last Updated**: August 20, 2026