Fix: prevent side panel from covering IDE when creating workflow with template

[lmac-1]

Description

Previously, SIDE_PANEL was at z-60 which covered the IDE at z-50, making it impossible to view job code when creating workflows from templates.

Lowering to z-30 is safe because:

• No UI elements exist between z-30 and z-50 that could conflict
• Template sidebar and AI assistant panel are mutually exclusive (they close each other when opened, so can't overlap)
• Side panels still correctly appear above canvas elements (z-5 to z-20)

This establishes the correct layering hierarchy:
Canvas (5-20) < Side panels (30) < IDE/Modals (50)

Closes #4347

Validation steps

1. Click "Create a new workflow" from Workflows page
2. Choose a template
3. Inspect the code of one of the steps

Before this PR it looked like this:

Expected: Code now sits on top of the sidebar rather than underneath

Additional notes for the reviewer

1. In the original issue there was also an error when you closed the AI assistant by the top right button, that it complained the job didn't exist. This has been fixed by something else since raising.
2. This PR made me realise that we have constants for z-index in the project. We definitely aren't using these and could be a potential clean up issue for a rainy day.

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[lmac-1]

Removing @doc-han as a reviewer and adding @elias-ba to review instead.

@elias-ba it's a small change, but you were the one who created the z-index constants file, so I think it makes sense for you to review. As I mentioned in the notes, it seems like we aren't using this file as we should and could be cleaned up for a rainy day later in the future, but would need solid regression testing.

[elias-ba]

Hey @lmac-1 this great! It's a clean and well-scoped fix. The z-index hierarchy Canvas (5-20) < Side panels (30) < IDE/Modals (50) makes sense.

One note for a future cleanup: there are ~15 hardcoded z-50/z-[60] values across the collaborative editor that don't use the Z_INDEX constants. Migrating those would help prevent this kind of layering issue from recurring, but that's a separate task, not a blocker here.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.39%. Comparing base (1f0ea49) to head (083d02e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4457      +/-   ##
==========================================
- Coverage   89.42%   89.39%   -0.04%     
==========================================
  Files         425      425              
  Lines       20221    20221              
==========================================
- Hits        18083    18076       -7     
- Misses       2138     2145       +7     

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