UX: Topic cards v2 (#173)

This is the second iteration on the topic card design, in which we bring
back the OP and change the layout.

**Changes**:
* Show OP avatar
* Remove activity avatar and replace by reply icon
* Remove activity icon background
* Move category tag to top left
* Replace long activity copy ("…replied at…") with dot separator
* Change date formatting to `tiny`
* Adjust bulk select styling to new layout + align checkbox to top on
mobile VS keep centred on desktop

* Why: On desktop, the avatar is taking 2 lines (usually) and aligning
the checkbox vertically looks nice. Exception for excerpts, but since
that's only available for pinned topics atm that's a low occurrence. On
mobile, the topic card is 3 lines, with a smaller avatar, which makes
the checkbox "float" around a bit when centred. Hence aligning it to the
top, which for solid avatars aligns nicely too.

* CSS refactor: grid, breakpoints

Messages/bookmarks have not been changed.

| Description | Visual |
|--------|--------|
| Large topic list | ![CleanShot 2025-06-02 at 17 37
35@2x](https://github.com/user-attachments/assets/f232058e-dbf6-4689-abe3-65970464a3e3)|
| Large bulk edit | ![CleanShot 2025-06-02 at 17 37
17@2x](https://github.com/user-attachments/assets/03d86b5f-d62b-4449-9c0b-452ceb8be60c)
|
| Medium topic list | ![CleanShot 2025-06-02 at 17 39
01@2x](https://github.com/user-attachments/assets/80739641-cf8f-4095-938f-9781cac57a7d)
|
| Medium bulk edit | ![CleanShot 2025-06-02 at 17 39
24@2x](https://github.com/user-attachments/assets/6e9045af-734a-4f3f-830a-e03a2f06fdd8)
|
| Small topic list | ![CleanShot 2025-06-02 at 17 39
44@2x](https://github.com/user-attachments/assets/eeca80bc-6863-4026-8f19-b1b5cf6848f3)
|
| Small bulk edit | ![CleanShot 2025-06-02 at 17 40
00@2x](https://github.com/user-attachments/assets/cde7be08-cde2-4ff2-b81b-328d3d7be848)
|
| Messages page (remains unchanged) | ![CleanShot 2025-06-02 at 17 20
37@2x](https://github.com/user-attachments/assets/6512bfe9-a699-4c67-b709-166540ee6fde)
|
| Bookmark page (remains unchanged) | ![CleanShot 2025-06-02 at 17 21
01@2x](https://github.com/user-attachments/assets/f8f43638-e911-49cb-a0d5-ebcb51ccfba4)
|

scss/desktop-horizon-fixes.scss

@@ -1,8 +1,8 @@
 // Fixing bulk select (only needed for desktop)
 .bulk-select-enabled {
-  .topic-list-header .topic-list-data.default {
-    position: sticky;
-    top: 10em;
+  .topic-list-header {
+    position: relative;
+    top: 0;
   }
 
   .topic-author-avatar-data {
@@ -47,19 +47,19 @@
     }
   }
 
+  // bulk select
   .bulk-select-topics {
     position: absolute;
-    right: -1em;
+    right: 0;
     background: var(--secondary);
     border-radius: 0 0 0 var(--d-border-radius);
-    padding: 1em;
-
-    @media screen and (max-width: 1048px) {
-      right: 0;
-    }
+    display: flex;
+    gap: 0.5rem;
+    margin: 0.5rem;
 
     button {
       white-space: nowrap;
+      margin: 0;
     }
   }
 }