refactor(app): split persistence helpers

[Astro-Han]

Summary

Split persistence helper logic out of packages/app/src/utils/persist.ts into focused read/migration and localStorage resilience modules.

Why

#687 identifies persist.ts as a first-batch frontend infrastructure debt file because it mixed storage targets, cache/quota eviction, legacy migration, and the Solid persisted wrapper in one >500 LOC utility. This PR keeps the public facade in place while moving the easiest independent helper responsibilities behind clearer file boundaries.

Related Issue

Refs #687.

Human Review Status

Pending. A human should make the final merge decision after reviewing the final diff and verification evidence.

Review Focus

• Confirm Persist, persisted, removePersisted, PersistTesting, and shouldDebugPersistedTerminalRead remain available from @/utils/persist.
• Confirm cache/quota/eviction/fallback behavior is only moved into persist-local-storage.ts, not changed.
• Confirm sync/async normalize and legacy migration behavior is only moved into persist-read.ts, not changed.
• Confirm this PR does not touch serialize.ts or active UI rewrite lanes.

Risk Notes

Behavior risk is low but broad: persistence is shared by layout, terminal, settings, language, server, model, prompt, and other app state. The change is intended to be behavior-preserving. No persistence keys, namespaces, data migrations, UI behavior, or desktop storage APIs changed.

Remaining #687 scope intentionally deferred: serialize.ts remains the next independent first-batch file. #601/#602/#604/#605/#606/#595/#615 lanes are not touched.

How To Verify

Focused tests: bun test --preload ./happydom.ts ./src/utils/persist.test.ts ./src/pages/layout/layout-page-store.test.ts -> 24 pass, 0 fail
Typecheck: bun --cwd packages/app typecheck -> passed
Frontend inventory: bun run frontend:inventory -> productionOver500 19 -> 18; warn-only inventory still exits 0
Diff check: git diff --check -> no whitespace errors
Code review: Requesting Code Review pass found no P0-P2 issues; P3 test naming was fixed before commit

Screenshots or Recordings

Not required. This is a non-visual persistence helper extraction.

Checklist

• Human review status is stated above as pending, approved, or not required
• I linked the related issue, or stated why there is no issue
• This PR has exactly one type label (bug, enhancement, task, or documentation), at least one primary routing label (app, ui, platform, harness, or ci), and exactly one priority label (P0 to P3), or I requested maintainer labeling
• I described the review focus and any meaningful risks
• I listed the relevant verification steps and the key result for each
• I did not introduce unrelated refactors, dependencies, generated files, or file changes beyond the stated scope
• I manually checked visible UI or copy changes when needed, with screenshots or recordings
• I considered macOS and Windows impact for platform, packaging, updater, signing, paths, shell, or permissions changes
• I called out docs, release notes, dependencies, permissions, credentials, deletion behavior, generated content, or local file changes when relevant
• I reviewed the final diff for unrelated changes and suspicious dependency changes
• I am targeting dev, and my PR title and commit messages use Conventional Commits in English

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: a8a0e465-4b27-4745-b2b6-12a27add9637

📥 Commits

Reviewing files that changed from the base of the PR and between 6a91751 and 2e87ebc.

📒 Files selected for processing (4)

• packages/app/src/utils/persist-local-storage.ts
• packages/app/src/utils/persist-read.ts
• packages/app/src/utils/persist.test.ts
• packages/app/src/utils/persist.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/i687-persist-split

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[github-actions]

Suggested priority: P2 (includes user-path files (packages/app/src/utils/persist-local-storage.ts, packages/app/src/utils/persist-read.ts, packages/app/src/utils/persist.test.ts, packages/app/src/utils/persist.ts)).

P1/P0 are reserved for maintainer confirmation. Please relabel manually if this is a release blocker, security issue, data-loss risk, or updater/runtime failure.

[gemini-code-assist]

Code Review

This pull request refactors the persistence logic by extracting local storage implementation and reading/migration utilities into dedicated files, persist-local-storage.ts and persist-read.ts. The changes introduce a caching layer with an eviction mechanism to manage storage quotas and include new tests for async data reading. Feedback was provided to optimize the key iteration in the eviction logic by replacing an unnecessary intermediate array with a standard for loop.

[github-actions]

Perf delta summary

Comparator: pass

Profile / Scenario	interaction median	interaction worst	long task max	tbt	frame gap p95	frame gap max	jank count	cls	status
default / homepage-cold	40 -> 32 (-8)	56 -> 40 (-16)	91 -> 60 (-31)	41 -> 10 (-31)	16.8 -> 33.4 (+16.6)	133.3 -> 116.7 (-16.6)	4 -> 3 (-1)	0 -> 0 (0)	pass
default / long-session-input-lag	48 -> 48 (0)	56 -> 48 (-8)	0 -> 0 (0)	0 -> 0 (0)	16.7 -> 16.7 (0)	16.8 -> 16.7 (-0.1)	0 -> 0 (0)	0 -> 0 (0)	pass
default / session-streaming-long	56 -> 40 (-16)	72 -> 64 (-8)	0 -> 0 (0)	0 -> 0 (0)	16.8 -> 16.8 (0)	16.8 -> 33.4 (+16.6)	0 -> 0 (0)	0 -> 0 (0)	pass
default / tool-call-expand	16 -> 16 (0)	16 -> 16 (0)	0 -> 0 (0)	0 -> 0 (0)	16.7 -> 16.8 (+0.1)	16.7 -> 16.8 (+0.1)	0 -> 0 (0)	0 -> 0 (0)	pass
default / tool-default-open-heavy-bash	32 -> 24 (-8)	32 -> 24 (-8)	63 -> 64 (+1)	13 -> 14 (+1)	33.4 -> 33.4 (0)	149.9 -> 166.6 (+16.7)	2 -> 1 (-1)	0 -> 0 (0)	pass
default / terminal-side-panel-open	48 -> 48 (0)	56 -> 48 (-8)	0 -> 0 (0)	0 -> 0 (0)	33.3 -> 33.3 (0)	33.4 -> 33.4 (0)	0 -> 0 (0)	0 -> 0 (0)	pass
default / session-scroll-reading	24 -> 16 (-8)	32 -> 24 (-8)	0 -> 0 (0)	0 -> 0 (0)	16.8 -> 16.8 (0)	16.8 -> 16.8 (0)	0 -> 0 (0)	0.505 -> 0.505 (0)	warn: cls