enhance(ui): tweak tooltip style and misc fixes (#38524)

Tooltip changes:

- center tooltips over their target (default placement `top` instead of
`top-start`)
- fix the arrow rendering off-center on references narrower than 22px
like 16px icons, caused by tippy's hardcoded 3px arrow padding
- fix arrowed tooltips overlapping their reference by 1px because the
offset distance was derived from the raw arrow option
- tweak tooltip colors and reduce content padding
- fix a Firefox bug where lazily created tooltips would not show on the
first hover, because Firefox skips enter/leave event dispatch when no
such listener existed in the window at the time of the pointer crossing

Other UI tweaks:

- tweak repo sidebar release style
- enhance actions back link
- center step chevron
- add border on workflow graph

<img width="92" height="76" alt="Screenshot 2026-07-21 at 18 07 42"
src="https://github.com/user-attachments/assets/968a51de-e470-48c0-b592-4a207dfcbb63"
/>
<img width="94" height="76" alt="Screenshot 2026-07-21 at 18 07 53"
src="https://github.com/user-attachments/assets/5e8e0ce3-7536-4689-a6ae-9cfb6e867ddb"
/>


<img width="203" height="77" alt="Screenshot 2026-07-19 at 00 14 32"
src="https://github.com/user-attachments/assets/71c16a54-ad14-4636-8af4-e68b3599f1d0"
/>
<img width="309" height="290" alt="Screenshot 2026-07-18 at 23 37 21"
src="https://github.com/user-attachments/assets/29630504-d69a-4e23-978b-41ae6d0bff92"
/>
<img width="151" height="110" alt="Screenshot 2026-07-18 at 23 35 47"
src="https://github.com/user-attachments/assets/b8107602-12f1-4f9b-a3f6-b930af25c9e7"
/>

---------

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

web_src/css/modules/tippy.css

@@ -52,7 +52,7 @@
 }
 
 .tippy-box[data-theme="tooltip"] .tippy-content {
-  padding: 0.5rem 1rem;
+  padding: 5px 10px;
 }
 
 .tippy-box[data-theme="tooltip"] .tippy-svg-arrow-inner,