feat: auto-trim schedule, TrimNow preview, settings rewrite with segmented tabs

[ricardoleal]

Description

Completes the auto-trim feature with daily schedule, adds a preview phase to TrimNow, rewrites Settings with a native macOS segmented tab layout, and fixes the window launch visibility bug.

• AutoTrimService — new service with daily timer (86400s), immediate eligibility check on start, Run Now bypasses toggle, cooldown uses autoTrimLastRun
• TrimUnit — extracted enum (days/weeks/months) with cutoffDate(byAdding:to:) and displayName(count:)
• SettingsStore — all 11 props converted from @ObservationIgnored+computed to stored var+didSet for proper @Observable tracking
• SettingsView — replaced TabView with custom segmented picker (Trim/Auto Trim/Helper/General tabs), card-based layout, removed all section header Labels and Picker "Unit" labels
• TrimNowSheet — configure → scanning → preview → deleting → done flow, user sees backup count before confirming deletion
• WindowSetup — orderOut(nil) when showWindowOnLaunch = false (fix: SwiftUI creates window before AppDelegate fires)
• BackupViewModel — prepareTrimNow() / confirmedTrimNow() split for preview flow; trimThresholdValue/Unit replaces ageThresholdMonths
• HelperClient — nonisolated isInstalled + uninstall() via osascript with admin privileges
• TrimByAgeView — unit picker, dynamic slider bounds per selected unit

Related Issue

#6

Type of Change

• Bug fix
• New feature
• Enhancement/improvement
• Documentation update
• Other (please describe):

How Has This Been Tested?

==> Compiling test sources...

==> Running tests...
TimeMachineTrimmer Tests
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## ResumptionFlag
  ✓ first call returns true
  ✓ second call returns false
  ✓ third call returns false
  ✓ only one succeeds from 100 concurrent calls

## Date Part Extraction
  ✓ full TM snapshot name
  ✓ non-TM string unchanged
  ✓ empty date part

## Backup Dict Validation
  ✓ valid dict returns nil
  ✓ empty snapshot returns error
  ✓ error mentions snapshotName
  ✓ empty volume returns error
  ✓ error mentions volumePath
  ✓ empty dict returns error

## HelperClient Backup Dict
  ✓ dict contains key 'id'
  ✓ dict contains key 'volumePath'
  ✓ dict contains key 'path'
  ✓ dict contains key 'snapshotName'
  ✓ Optional("my-id") == Optional("my-id")
  ✓ Optional("/path with/spaces") == Optional("/path with/spaces")
  ✓ Optional("snap") == Optional("snap")
  ✓ Optional("/Volumes/My Disk") == Optional("/Volumes/My Disk")
  ✓ empty id ok
  ✓ 4 keys even with empty values

## XPC Reply Format
  ✓ empty string means success
  ✓ non-empty string means error
  ✓ success = empty
  ✓ failure = error message


================================================
Integration Tests
================================================

-- Build verification --
  ✓ binary is Mach-O format
  ✓ binary is arm64 architecture
  ✓ code signature present

-- XPC communication --
  ✓ ping: helper is alive
  ✓ version: XPC response received

-- Delete operations --
  ✓ delete: returned (no hang)
  ✓ delete: produced a result line
  ✓ delete (bad volume): returned (no hang)

──────────────────────────────
35 passed, 0 failed
──────────────────────────────

Checklist

• My code follows the existing code style
• I've tested the changes on macOS 26 (Tahoe)
• I've updated the README if necessary
• I've added comments for complex logic
• My changes don't break existing functionality

Screenshots (if applicable)

## Additional Notes