fix: Back to home href on dashboard error pages in preview mode

[royendo]

• The Back to home button on ErrorPage instances in the web-local explore and canvas pages defaulted to /, which is the developer-mode home. In preview mode the home is /dashboards, so users landed in developer mode after errors like Error building dashboard.
• Derive the href from previewModeStore on both pages, matching the pattern already used in +error.svelte.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

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Developed in collaboration with Claude Code