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>

docs/users.md

@@ -8,7 +8,7 @@ fetch, one parse and one file.
 
 | Yours alone | The same for everyone |
 |---|---|
-| Read, kept, playback position | The feed's URL |
+| Read, pinned, playback position | The feed's URL |
 | Which feeds you see at all | Its download folder |
 | Keywords, auto-download, explicit, per-scan cap | When it is scanned |
 | | The file on disk |
@@ -34,10 +34,10 @@ is shared.
 
 Deleting a file deletes everyone's copy. A feed with other subscribers labels the button **Delete
 for everyone** and names them in the confirmation, and the server has the last word: if anyone else
-has kept the item or not played it yet, `DELETE /api/enclosures/{id}` answers `409` with the
+has pinned the item or not played it yet, `DELETE /api/enclosures/{id}` answers `409` with the
 reason, and only `?force=true` goes through.
 
-Retention follows the same rule: an item anyone kept keeps its file, and it counts as read only once
+Retention follows the same rule: an item anyone pinned keeps its file, and it counts as read only once
 every subscriber has read it.
 
 ## Signing in