Fix #8738, #8708: Fix memory leak in `SettingsViewController`

[StephenHeaps]

Summary of Changes

This pull request fixes #8738, fixes #8708

Submitter Checklist:

• [x] Unit Tests are updated to cover new or changed functionality
• [x] User-facing strings use NSLocalizableString()
• [x] New or updated UI has been tested across:
  • [x] Light & dark mode
  • [x] Different size classes (iPhone, landscape, iPad)
  • [x] Different dynamic type sizes

Test Plan:

1. Restore a wallet with NFTs
2. Once Portfolio is shown, dismiss wallet
3. Tap ... -> Settings -> Web3 (must be done through browser settings)
4. Reset wallet
   • Do not quit / relaunch the app
5. Re-open wallet and restore the same wallet from step 1
6. Open NFT tab, enable auto-discovery and wait
7. Observe no assets are duplicated, NFTs displayed in a grid as expected

Screenshots:

Reviewer Checklist:

• [ ] Issues include necessary QA labels:
  • QA/(Yes|No)
  • bug / enhancement
• [ ] Necessary security reviews have taken place.
• [ ] Adequate unit test coverage exists to prevent regressions.
• [ ] Adequate test plan exists for QA to validate (if applicable).
• [ ] Issue and pull request is assigned to a milestone (should happen at merge time).