fix: enable more djlint rules and improve keyboard navigation (#40081)

Enable djlint's [H018](https://djlint.com/docs/linter/#h018),
[H046](https://djlint.com/docs/linter/#h046) and
[H054](https://djlint.com/docs/linter/#h054) rules, fix their findings
and the keyboard navigation problems around them. H054 found links
nested in `<button>` dropdowns and radio inputs nested in links, both
invalid HTML.

- "Add File" and "More Operations" in the repo file list become `div`
dropdowns, the disabled state stays unfocusable
- The diff whitespace menu marks the current option with a check and
`aria-current` instead of radios, which made our dropdown aria patch
announce it as a combobox
- Sign-in moves "Forgot password?" below "Sign In" and drops the
positive tabindexes from https://github.com/go-gitea/gitea/pull/31689,
so the tab order follows the DOM: username, password, remember, sign in,
forgot
- The focus ring on primary, red and green buttons is near-white on dark
themes and near-black on light ones, the primary-colored ring was
invisible on primary buttons

<img width="500" alt="Whitespace menu, before and after"
src="https://github.com/user-attachments/assets/d22a423d-95e5-434c-aec4-59264c7132b7"
/>

<img width="600" alt="Sign-in form, main and this PR"
src="https://github.com/user-attachments/assets/d6780690-d906-4791-8a2b-8d80fb03067a"
/>

<img width="215" height="132" alt="Keyboard-focused primary and red
buttons, dark and light theme"
src="https://github.com/user-attachments/assets/a2400b18-4802-4383-8560-7e8267a7f02f"
/>

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/css/base.css

@@ -331,11 +331,21 @@ a.suppressed:hover {
 a:focus-visible,
 button:focus-visible,
 .ui.button:focus-visible,
+.ui.dropdown:not(.selection):focus-visible,
 [role="button"]:focus-visible {
   outline: 2px solid var(--color-primary);
+}
+
+button:focus-visible,
+.ui.button:focus-visible {
   outline-offset: -2px;
 }
 
+.ui:is(.primary, .red, .green).button:focus-visible,
+.ui:is(.primary, .red, .green).buttons .button:focus-visible {
+  outline-color: var(--color-text-dark);
+}
+
 a.silenced:hover {
   text-decoration-line: none;
 }