feat(pulls): support per-commit review (#39326)

Opening a single commit of a PR let you look, but not review. The Review
button was disabled and there was no way to reach the next or previous
commit. This finishes https://github.com/go-gitea/gitea/pull/37475,
which only reached the head commit.

- Reviews can be submitted from a single commit, a commit range stays
disabled
- A bar above a single-commit diff shows the commit, its position in the
PR and prev/next buttons
- The commit dropdown marks the commits on screen
- Timeline commits open the PR's commit view, commits force-pushed out
of the PR redirect to the repository commit page

Assisted-by: Claude Code (Fable 5.1)
Co-authored-by: bircni <bircni@icloud.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

web_src/css/modules/button.css

@@ -32,7 +32,11 @@
   color: var(--color-text);
 }
 
-.ui.button.hover-opaque:hover {
+.ui.ui.ui.buttons.hover-opaque {
+  background: var(--color-body)
+}
+
+.ui.ui.ui.button.hover-opaque:hover {
   /* ".ui.button:hover" uses alpha channel to "highlight", it doesn't work when the background is not body: the inherited color is not right
    for such case, use this opaque color */
   background: var(--color-hover-opaque);
@@ -282,6 +286,7 @@
   font-size: 0;
   vertical-align: baseline;
   margin: 0 0.25em 0 0;
+  border-radius: var(--border-radius);
 }
 
 /* btn is a plain button without any opinionated styling, it only uses flex for vertical alignment like ".ui.button" in base.css */