refactor: extract populateEditorFields to eliminate duplicated form setup

[JuliaKalder]

Summary

• Extracted populateEditorFields(template) helper in options/options.js (after the module-level variable declarations), handling both a template object and null for blank forms
• openEditor (when editing an existing template) now calls populateEditorFields(template) instead of duplicating 20+ lines of form-setup code
• duplicateTemplate now calls populateEditorFields(template) then overrides nameInput.value with the i18n-prefixed duplicate name (the only intentional difference between the two callers)
• Removed the now-redundant pendingAttachments pre-assignment in duplicateTemplate (the helper sets it)
• No behavior changes: all 90 previously-passing tests still pass; the 7 pre-existing migrateV0toV1 failures are unrelated to this change

Verification

• Ran npm test: 90 pass / 7 fail (same 7 pre-existing failures in template-store.test.js::migrateV0toV1, unrelated to options.js)
• Diff: 37 insertions / 55 deletions — net removal of ~18 lines of duplicated code

Fixes #79

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