feat(settings): split backup and tuning panels

[cooper (czxtm)]

Summary

Test Plan

Docs

• Docs updated (companion PR in darkmatter/nixmac-web: #___)
• No docs update needed

[cooper (czxtm)]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
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• refactor(frontend): move backend mirrors to viewmodel #255
• chore(beads): update state migration issues #254
• chore(fmt): add treefmt wiring #253
• feat(console): add resizable log panel #252
• fix(theme): scope Minted to Monaco diffs #251
• feat(settings): split backup and tuning panels #250 👈 (View in Graphite)
• chore(credentials): remove legacy plaintext fallback #249
• feat(state): wire runtime slices and summarizer #248
• feat(db): add Diesel pool and table models #247
• feat(state): add scoped slice persistence #246
• refactor(configurable): split derive and generated UI #245
• docs(state): add migration plan issues #244
• implement derive(Configurable) macro #228 : 1 other dependent PR (#229 )
• develop

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[github-actions]

	Warnings
⚠️	Your ## Test Plan section is empty. Describe the steps a reviewer should take to verify this change.
⚠️	New UI components were added without a Storybook story. Consider adding a sibling *.stories.tsx file: apps/native/src/components/widget/settings/backup-restore-section.tsx apps/native/src/components/widget/settings/tuning-tab.tsx

📋 PR Overview

Lines changed	675 (+498 / -177)
Files	4 added, 9 modified, 0 deleted
Draft / WIP	no
Has Test Plan	yes
New UI components	yes (2)
New Storybook stories	no
New Rust modules	no
New TS source files	yes (3)
New tests	yes (1)
package.json touched	no
Cargo.toml touched	no
Infra / CI touched	no

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