Apply min-width to the confirmation pages

**What is this about?**

[Needs design confirmation] reconsider min-width

Following the work to align responsiveness across various confirmation pages, we'd like to add a min-width of either 320px (standard min-width device support) or 400px (default popup width).

Centering of confirmations needs refactor since shrinking causes overflow on both left and right. In these cases, the overflow: auto does not trigger auto scrolling.

Spec:
- Send
- Redesign Confirmation Pages
- Install Snap flow
- Various Snap pages (See https://metamask.github.io/snap-simple-keyring/latest/ using flask mode)
- Account (Dapp) Permissions

**less than min-width 320px**

**Stakeholder review needed before the work gets merged**

- [ ] Engineering (needed in most cases)
- [ ] Design
- [ ] Product
- [ ] QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- [ ] Security
- [ ] Legal
- [ ] Marketing
- [ ] Management (please specify)
- [ ] Other (please specify)

Start by locating the confirmation-page implementations for Send, Redesign Confirmation Pages, Install Snap, the listed Snap pages, and Account (Dapp) Permissions. Confirm with design whether the minimum width is 320px or 400px, then verify centering and overflow behavior below that width; done requires stakeholder review before merge.

Written by the indexing model from the issue text.