fix: enforce identity restrictions on nested template resolution

[JuliaKalder]

Summary

• resolveNestedTemplates was looking up {{template:Name}} includes from ALL templates, ignoring their identities restrictions — identity-scoped templates could be silently included by any other template.
• In insertTemplateIntoTab, the current compose identity is now fetched at the top of the function via messenger.compose.getComposeDetails(tabId).
• allTemplates is filtered to allowedTemplates before the lookup maps are built: a template passes if its identities field is absent/empty (unrestricted) or explicitly lists the current identity.
• The filtered maps are passed to resolveNestedTemplates, so nested includes can only resolve templates the active identity is permitted to use.

Test plan

• All existing template-insert.test.js tests continue to pass (confirmed locally — 7 pre-existing failures in template-store.test.js migrateV0toV1 are unrelated to this change).
• Manually: compose a message with identity A, insert a template containing {{template:RestrictedForB}} where RestrictedForB.identities = ["identityB"] — the include should not resolve (warn and leave marker or skip).
• Verify that templates with no identities restriction still resolve normally for any identity.
• Verify templates explicitly listing the current identity still resolve correctly.

Fixes #48