Show "Fork from here" button on the mobile chat

[kevin-dp]

Follow-up to #4410: extends the per-message "Fork from here" affordance to the mobile chat (and any other surface that mounts agents-server-ui via an Expo "use dom" embed).

What changes

Two source files, both in packages/agents-server-ui:

1. src/components/views/ChatView.tsx — ChatLogView (the view the mobile embed mounts under view='chat-log') now computes the same forkFromHereByInboxKey map that ChatView already had on the web side. It pulls db from useEntityTimeline, forkEntity from useElectricAgents, walks visibleRows to find the latest preceding completed runs row's pointer, and threads the per-row callback to EntityTimeline. Without this, UserMessage rendered in the mobile embed never received an onForkFromHere prop and the button stayed off the DOM regardless of CSS.

2. src/components/UserMessage.module.css — adds a touch-targeted reveal rule scoped via :global(html[data-electric-mobile-dom='true']) .forkButton { opacity: 1 }. Touch devices don't fire :hover, and .bubble:focus-within only kicks in after a tap, so without the override the button stayed invisible until interaction. The reveal lives inside the CSS module (rather than a sibling stylesheet that targets the hashed class by substring) so whichever pipeline is in play — Vite for the web/desktop bundles, Metro for the Expo DOM embed — hashes .forkButton consistently with the JSX.

The mobile package itself doesn't change. The embed already mounts ChatView → EntityTimeline → UserMessage, the fork POST already routes through serverFetch() running in the embed's JS context, and post-fork navigation already routes through onRequestOpenEntity → openSession(target). Once ChatLogView produces the callback map and CSS makes the button visible, the whole flow lights up.

Why split from #4410

#4410 was framed as the web/desktop release. The mobile surface depends on the same wiring but ships separately so the web work can land without being held by mobile-specific UX polish. Once both merge, the affordance is uniform across web, desktop, and mobile.

Test plan

• Build + run Fork at message: re-roll a session from an earlier point in its history #4410's base. Start the mobile app with pnpm -C packages/agents-mobile ios.
• Open any session with at least two completed runs.
• The second user-message bubble shows the GitFork icon at full opacity, top-right corner of the bubble.
• Tap it. The app navigates to a new entity URL ending in -fork-<hash>; the new session contains only the first exchange.
• Send a new prompt in the fork; it runs independently.
• First user message in a chat shows no button. While Horton is streaming a reply, the triggering message shows no button. Button returns after ✓ done.
• On desktop and web, hover-reveal still works as before (the mobile rule is scoped via the data-electric-mobile-dom attribute the embed sets).

🤖 Generated with Claude Code

[github-actions]

Electric Agents Desktop Builds

Build artifacts for commit b30d2c9.

Platform	Status	Artifact
macOS Apple Silicon	Passed	DMG
macOS Intel	Passed	DMG
Windows x64	Passed	Installer
Linux x64	Passed	AppImage / deb

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 35.47%. Comparing base (0be3760) to head (5e2d693).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...agents-server-ui/src/components/views/ChatView.tsx	0.00%	19 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (0be3760) and HEAD (5e2d693). Click for more details.

HEAD has 24 uploads less than BASE

Flag	BASE (0be3760)	HEAD (5e2d693)
unit-tests	10	2
packages/y-electric	1	0
typescript	10	2
packages/start	1	0
packages/agents-mcp	1	0
packages/electric-ax	1	0
packages/agents	1	0
packages/react-hooks	1	0
packages/experimental	1	0
packages/typescript-client	1	0

Additional details and impacted files

@@                    Coverage Diff                     @@
##           kevin/fork-at-message    #4413       +/-   ##
==========================================================
- Coverage                  47.36%   35.47%   -11.90%     
==========================================================
  Files                        253      184       -69     
  Lines                      19797    13822     -5975     
  Branches                    6230     4674     -1556     
==========================================================
- Hits                        9377     4903     -4474     
+ Misses                     10401     8904     -1497     
+ Partials                      19       15        -4     

Flag	Coverage Δ
packages/agents	?
packages/agents-mcp	?
packages/agents-server	73.49% <ø> (ø)
packages/agents-server-ui	6.16% <0.00%> (-0.02%)	⬇️
packages/electric-ax	?
packages/experimental	?
packages/react-hooks	?
packages/start	?
packages/typescript-client	?
packages/y-electric	?
typescript	35.47% <0.00%> (-11.90%)	⬇️
unit-tests	35.47% <0.00%> (-11.90%)	⬇️

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[msfstef]

can probably have smaller comments

[github-actions]

Electric Agents Mobile Build

Android preview build for commit b30d2c9.

Platform	Profile	Status	Build
Android	preview	Passed	EAS build

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