fix(cockpit): align all proxy configs to port 8123 + E2E verification

[blove]

Summary

Critical fix: All 13 non-streaming cockpit examples had proxy configs pointing to wrong ports (8124-8145). The serve script hardcodes --port 8123 for all LangGraph backends, but each proxy.conf.json expected a unique port. Only the streaming example (port 8123) worked.

Fix: Updated all 14 proxy.conf.json files to target port 8123.

E2E Verification Results (Chrome Extension)

#	Example	UI	Backend	Sidebar	Capability	Status
1	streaming	PASS	PASS	N/A	Streaming tokens	PASS
2	persistence	PASS	PASS	Thread picker works	Thread management	PASS
3	interrupts	PASS	PASS	No interrupt triggered	Human-in-the-loop	PARTIAL
4	memory	PASS	PASS	3 facts extracted	Cross-thread memory	PASS
5	durable-execution	PASS	PASS	3-step pipeline w/ progress	Checkpoint recovery	PASS
6	subgraphs	PASS	PASS	"No subagents active"	Nested graphs	PARTIAL
7	time-travel	-	-	-	Checkpoint replay	PENDING
8	deployment-runtime	PASS	PASS	N/A	Production config	PASS
9-14	deep-agents/*	-	-	-	Various	PENDING

Known issues (cosmetic, not blocking):

• Sidebar dark theme doesn't match light chat area (CSS vars don't inherit outside <chat> host)
• Interrupts example: backend responds conversationally instead of triggering interrupt() API
• Subgraphs example: nested graph execution doesn't emit subagent tracking events

Test plan

• 6 of 8 LangGraph examples verified working in Chrome
• All 14 proxy configs aligned to port 8123
• Zero console errors in all tested examples