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>

README.md

@@ -12,7 +12,7 @@ behind iPodderX (2004-2008, Ray Slakinski & August Trometer).
 - **The web UI.** It has a toolbar, and a feed list that opens with Directory, Popular and All
   Subscriptions. Items sit in a sortable table with a Files pane, and there is a player bar. It
   comes in Dark, Light and Classic themes, and works on a phone.
-- **Several people, one copy.** Each person has their own subscriptions and their own read, kept
+- **Several people, one copy.** Each person has their own subscriptions and their own read, pinned
   and playback state. There is one file on disk per episode, however many people want it. People
   sign in with a password or through a proxy (Cloudflare Zero Trust or Authentik), and admins
   manage accounts and settings.
@@ -21,7 +21,7 @@ behind iPodderX (2004-2008, Ray Slakinski & August Trometer).
   on new downloads per scan.
 - **Downloads.** Files come over HTTP or BitTorrent and are filed into a folder per feed.
   Retention deletes the oldest files to stay under a disk quota or an age limit, and never touches
-  an item someone has kept.
+  an item someone has pinned.
 - **OPML.** You can import and export your own subscriptions. You can also subscribe to an OPML
   URL, which keeps a whole list in step as a folder.