Pin, not flag

Keeping an item is pinning it now: a thumbtack where the flag was, and
Pin, Pinned and Unpin where Keep, Kept and Stop keeping were, on the
toolbar, the item's own buttons, the filter tab, the table column, the
retention hint and the warning before deleting a shared file. Pinned is
the solid thumbtack and not pinned the same shape outlined, as the flag
had its regular and solid pair. The API and database keep `flagged`.

The icon test compared glyphs by their path alone, which the two pins
share; it compares the whole glyph now.

Closes #9.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/web.rs

@@ -1259,9 +1259,9 @@ async fn delete_file(
         let complaint = match (starred, unread) {
             (0, 0) => None,
             (0, u) => Some(format!("{} subscribed to this feed {} not played it yet", people(u), if u == 1 { "has" } else { "have" })),
-            (st, 0) => Some(format!("another {} kept it", people(st))),
+            (st, 0) => Some(format!("another {} pinned it", people(st))),
             (st, u) => Some(format!(
-                "another {} kept it, and {} not played it yet",
+                "another {} pinned it, and {} not played it yet",
                 people(st),
                 if u == 1 { "one person has".to_string() } else { format!("{u} have") }
             )),