Phone reader: back and the item's actions float over the text (#456)

On a phone they sat above the title and scrolled away with it, so
marking, pinning or sharing a long post meant scrolling back to its
top. Now back sits alone on the left and the actions in a capsule on
the right, sticky at the top of the reader, as Gmail's iOS reader does.
Each theme's own panel, frosted, so no theme changes colour; Glass gets
its sheen and a denser fill, since the text scrolls right under them,
and Classic its toolbar metal. Read and pin redraw the reader, which
now keeps its scroll position, since they are pressed mid-post. The
desktop reader is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RqkrVA8TdTxe556JB3sR1n

web/CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- On a phone, back and the item's actions (read, pin, open, share) float over the text as it scrolls, so they are at hand anywhere in a long post without scrolling back to the top; pressing one there keeps your place (#456).
 - Share on an episode offers Share link, Share from where you are, and Copy link, with a link to the episode's page on ipx-play that anyone can play; an episode of a private feed offers its own page in the same menu, never its file, which a paid feed signs for you, and says why there is no page anyone can play. An admin sets ipx-play up in Admin, Directory (#447, #452, #454, #455).
 - In the iOS app, Settings has a This device section for the app's server and player token, with a button that makes a token and hands it to the app (#445).
 - In the iOS app, the page registers the phone with ipx for its icon's count, and signing out removes it (#433).