Fix submit button overflow on auth form cards (fieldset min-width)

[mdon]

Summary

Surfaced while testing the projects-side AI Translate work — the magic-link form's submit button visibly stuck out past the card on the right after submit, with text wrapping awkwardly across three lines.

Two root causes, both fixed:

1. <fieldset> browser-default min-width

<fieldset> carries a browser-default min-width: min-content. When a long-content child (the icon + Send Magic Link → button) had a wider intrinsic size than the form, the fieldset grew past the form's width, and the button's w-full honored the wider fieldset — overflowing ~100px past the card edge.

Fix: min-w-0 on the fieldset. Two of eight auth forms (magic_link_registration, registration) already had this exact fix — someone hit this bug class before and patched those two but missed the other six. Applied uniformly across all six remaining: confirmation, confirmation_instructions, forgot_password, login, magic_link, reset_password.

2. phx-disable-with ↔ @loading branch collision (magic-link only)

The magic-link form had both phx-disable-with="Sending magic link..." AND an <%= if @loading do %> branch in the button content. Both mechanisms swap the button content on submit. The double-swap confused Phoenix LV's diff merger, which injected a stray empty <svg data-phx-skip> element from the post-submit success-alert icon into the button (default SVG dims = 300×150, computed to ~183×150 in this layout). That squeezed the real text to wrap onto three lines.

Fix: dropped phx-disable-with. The @loading branch already renders the spinner + "Sending magic link..." state. Also added min-w-0 max-w-full to the button itself as belt-and-suspenders.

Test plan

Verified in Chrome at /phoenix_kit/users/magic-link:

• Initial render: button is exactly form-width (336px), text on one line
• Post-submit: button stays form-width, "✉ Send Magic Link →" still on one line in disabled state
• Success alert renders cleanly below the button
• No phantom <svg data-phx-skip> in the button DOM
• No JS console errors