Update flash messages

[lleger]

• Look over your code one last time.
• Test all functionality you expect to be QA'd. Don’t forget to QA user interface changes in the browser.
• Describe what changes this PR includes.
• Post screenshots for design review if applicable.
• Fill in QA Notes listing clear, ordered steps for testers.

Description

Update flash messages

• Make them human readable by default
• Restyle with Tailwind
• Use gettext for strings
• Use new Phoenix components instead of content_tag

Closes [sc-18131].

Update CSP to not include subdomain
By default, let's leave it just as the root domain, since that is what
we'll use by default. If an app needs a subdomain, it can be added in
that app specifically -- but this is a business decision, not a general
one.

Screenshots

QA Notes

PR Author Browser Info: Safari

**QA Tester Browser Info:**

## Environment Setup

- [ ] [Add necessary environment setup, e.g. whether to update dependencies, OR specify none]

## Testing Steps

- [ ] Sign up for an account or reset a password — any action to trigger a flash

## Notes

- [ ] [Tester should add any comments, questions, or concerns that pop up during QA]

[shortcut-integration]

This pull request has been linked to Shortcut Story #18131: Human readable error messages by default.

[lleger]

This will need to be rebased once #395 is merged.