Rework checkbox styling, remove input border hover effect (#36870)

- Rework all checkbox styling to be consistent inside and outside
markup.
- Remove `input` border hover effect. Was too subtle and honestly
unneeded, consistent with GitHub.
- Increase `input` border contrast slightly.
- Some small spacing fixes in Markup (nested tasklist and spacing after
checkbox).

<img width="221" height="222" alt="Screenshot 2026-03-09 at 08 18 19"
src="https://github.com/user-attachments/assets/9e66abee-7102-4abe-9b00-e3f9b24ed735"
/>
<img width="226" height="217" alt="Screenshot 2026-03-09 at 08 18 10"
src="https://github.com/user-attachments/assets/33cdac26-4479-41da-9488-e60d70c5c997"
/>
<img width="79" height="218" alt="Screenshot 2026-03-09 at 08 17 32"
src="https://github.com/user-attachments/assets/ae1064a2-2bb3-44e7-a00b-2f4f5aad4241"
/>
<img width="267" height="297" alt="Screenshot 2026-03-09 at 08 17 07"
src="https://github.com/user-attachments/assets/1237fa98-0d94-4023-a87d-190d89c57421"
/>
<img width="558" height="260" alt="Screenshot 2026-03-09 at 08 21 04"
src="https://github.com/user-attachments/assets/1908a794-3394-494c-b2d5-470c00c668d1"
/>

---------

Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

web_src/css/themes/theme-gitea-dark.css

@@ -205,8 +205,7 @@ gitea-theme-meta-info {
   --color-input-text: var(--color-text-dark);
   --color-input-background: #171a1e;
   --color-input-toggle-background: #2e353c;
-  --color-input-border: var(--color-secondary);
-  --color-input-border-hover: var(--color-secondary-dark-1);
+  --color-input-border: var(--color-secondary-dark-1);
   --color-light: #00001728;
   --color-light-mimic-enabled: rgba(0, 0, 0, calc(40 / 255 * 222 / 255 / var(--opacity-disabled)));
   --color-light-border: #e8f3ff28;