Fix navigation for custom editor paths

[cstns]

Description

extends on #7783

Adds a useUrlHelper composable (safeOrigin, originsMatch) that wraps new URL().origin in a try/catch so a malformed URL can't throw and break message handling.

Wires it into the two places that compare postMessage origins:

• HostedInstanceEditorWrapper.vue was doing a raw === between event.origin (bare origin) and instance.url (can include a path from httpAdminRoot), so the comparison would silently fail for non-root editor paths. Same root cause as Expert panel drops nr-assistant editor messages as "unknown origin" on instances with a non-root httpAdminRoot #7782 but in the iframe message listener rather than the Expert panel.
• product-assistant.js already had the fix from fix: normalise Expert inbound origins to bare origin #7783 with an inline new URL().origin, now uses the shared composable instead.

Related Issue(s)

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
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Labels

• Includes a DB migration? -> add the area:migration label

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.04%. Comparing base (0d4cf33) to head (9c5e6aa).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7784      +/-   ##
==========================================
- Coverage   75.05%   75.04%   -0.02%     
==========================================
  Files         429      429              
  Lines       22714    22777      +63     
  Branches     6012     6036      +24     
==========================================
+ Hits        17048    17092      +44     
- Misses       5666     5685      +19     

Flag	Coverage Δ
backend	75.04% <ø> (-0.02%)	⬇️

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[andypalmi]

Looks good. One thing to fix before merge:

• UrlHelper.ts is missing a trailing newline, which fails the eol-last lint rule.

Non-blocking suggestion: useUrlHelper has no reactive state or lifecycle, so it's a plain function factory rather than a composable. Might fit better under utils/ alongside the other URL helpers.

[cstns]

Non-blocking suggestion: useUrlHelper has no reactive state or lifecycle, so it's a plain function factory rather than a composable. Might fit better under utils/ alongside the other URL helpers.

that's a good observation! I'm still undecided if we should keep both composables and utils or merge them into one