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>

CHANGELOG.md

@@ -13,7 +13,7 @@ The long form, with what was wrong before and how it was found, is in
 ### Added
 
 - Keyboard shortcuts after Feedly's: j and k through items, Shift-J and Shift-K through feeds,
-  g and a letter to go to a place, o to play, s to keep, and more. Press ? for the whole list.
+  g and a letter to go to a place, o to play, s to pin, and more. Press ? for the whole list.
 - Directory can be filtered to Podcasts or Blogs, and by each show's own iTunes category as a row
   of chips, the narrower one where a show gives two (Games, not Leisure). The two combine, and
   both filter in place.
@@ -22,6 +22,8 @@ The long form, with what was wrong before and how it was found, is in
 
 - Directory shows each feed as its cover art in a grid, title and subscriber count underneath,
   instead of a list. Popular and the Add a feed dialog keep their rows.
+- Keeping an item is now pinning it: a thumbtack in place of the flag, and Pin, Pinned and Unpin
+  in place of Keep, Kept and Stop keeping. A pinned item is still never deleted.
 - Currently Listening is its own place in the feed list, below Popular, instead of a section at
   the bottom of the Popular page.
 - The first scan after upgrading fetches every feed in full once, on its usual schedule, so each