Add Priority 5 Frontend Architecture Decoupling Checklist

[rendall]

Motivation

• Provide a concrete implementation checklist to decouple auth and login behavior from view components so authentication and session lifecycle are not tied to Login.svelte presence.
• Clarify boundaries between view components, state machines, and auth workflows to make UI modules easier to test and future upgrades less error-prone.
• Capture scope, success signals, and validation requirements for a Priority 5 repo-health effort focused on login/auth surfaces.

Description

• Add docs/plans/Priority5FrontendArchitectureDecouplingChecklist.md containing atomic work items C01–C08 to extract auth storage, workflows, controller, stores, runtime, and component refactors.
• Include validation items T01–T03, behavior slices (A–E), dependency traces, success targets, and conformance QC notes to guide incremental implementation.
• Split consumer migration into separate atomic items for CommentInput and SelfDisplay (C07 and C08) and document clear dependencies and test mapping to minimize behavioral drift.

Testing

• No automated tests were run for this change because it is a documentation-only addition; CI tooling may lint or validate Markdown separately but no repo tests were executed as part of this PR.

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Codex Task