refactor: reorganize security settings by domain

[GFrancV]

Summary

• Part 1/2 of Reorganize Security Settings: fix semantic grouping and scale beyond a single scrolling modal #6 — pure reorganization, no new components/ui/ primitives, no changes to KeyFileSetting's internal wizard logic (that extraction into its own dialog is part 2).
• Gear dropdown: moved "Open backups folder" and consolidated it with "Vault Location" (path, save copy as); removed "Auto-lock" timeout and "Change password" from here — both now live in Security settings.
• Security settings modal: unified Auto-lock (timeout + "Lock when minimized" together); added an "Authentication" section grouping Password (opens ChangePasswordDialog, now owned by this dialog) and Key file with equal visual weight; removed the Backups section (moved to dropdown); condensed the 3 previously-readonly checkbox rows into one "Always-on protections" info paragraph.
• "Change password" now closes Security settings before opening (no dialog stacking — matches the rest of the app's one-dialog-at-a-time pattern).

Test plan

• pnpm validate (typecheck + lint + format) passes
• Manually verified in pnpm dev: auto-lock timeout persists, dropdown shows the new grouping, backups folder opens, change-password flow closes/opens correctly, key file wizard still works in its new location, remaining toggles persist

[github-actions]

React Doctor found 5 issues in 2 files · 5 warnings · score 80 / 100 (Needs work) · vs main

5 warnings

src/renderer/src/components/settings/KeyFileSetting.tsx

• ⚠️ L30 Large component is hard to read and change no-giant-component
• ⚠️ L30 Many related useState calls prefer-useReducer
• ⚠️ L84 Loading flag reset outside finally no-loading-flag-reset-outside-finally
• ⚠️ L103 Loading flag reset outside finally no-loading-flag-reset-outside-finally

src/renderer/src/components/sidebar.tsx

• ⚠️ L343 Array lookup inside a loop js-set-map-lookups

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_{Reviewed by React Doctor for commit 6d2a6cd. See inline comments for fixes.}