Show which step an AI edit changed

[hanna-paasivirta]

Description

In user testing, people found it hard to tell where the AI's edits landed. This PR makes the AI's code diffs point clearly to the step they changed.

• The diff header shows the step name next to its adaptor logo.
• Click the step name or "Go to step" to open the step. The code the AI added flashes twice, then stays lightly highlighted until you click or type.
• The highlight finds the code by its text, so it still lands in the right place if the step was edited since.
• Clicking a removed step gives it a small shake, with a "This step was removed" tooltip.
• The diff shows one column of line numbers instead of two.
• Added steps with only placeholder code start folded, with no Copy button.

Closes BLD-99. Follows on from #5036.

Validation steps

1. Ask the AI assistant to change a step's code.
2. Click the step name in the diff. The step should open with the new code highlighted, flashing twice.
3. Click it again. The highlight should flash again.
4. Click or type in the code. The highlight should go away.
5. Ask the assistant to remove a step, then click its name. It should shake and not open.

Additional notes for the reviewer

1. Not yet checked in a browser. Tests pass.

AI Usage

Please disclose whether you've used AI anywhere in this PR (it's cool, we just want to know!):

• I have used Claude Code
• I have used another model
• I have not used AI

You can read more details in our Responsible AI Policy

Pre-submission checklist

• I have performed an AI review of my code (we recommend using /review with Claude Code)
• I have implemented and tested all related authorization policies. (e.g., :owner, :admin, :editor, :viewer)
• I have updated the changelog.
• I have ticked a box in "AI usage" in this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.6%. Comparing base (aac8674) to head (bf8543f).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5250     +/-   ##
=======================================
+ Coverage   91.4%   91.6%   +0.2%     
=======================================
  Files        465     465             
  Lines      23416   23416             
=======================================
+ Hits       21399   21444     +45     
+ Misses      2017    1972     -45     

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[github-actions]

Security Review ✅

• S0 (project scoping): N/A — all changes are React/TypeScript, CSS, docs, and vitest tests under assets/js/collaborative-editor/ and assets/js/monaco/; no new queries or web-layer entrypoints touch project-scoped data.
• S1 (authorization): N/A — no controller actions, LiveView handle_events, or policy modules are added or modified; the UI operates on data already loaded via the existing authorized channel.
• S2 (audit trail): N/A — no Ecto.Multi / Repo.insert|update|delete on config resources is added; this PR only renders diff highlights and step pills client-side.