Admin page laid out as a feed's settings are (#414)

Prototype for approval on the issue; not to be merged until then.

The page had grown one captioned box at a time: at 390 px the tabs ran off
the edge and hid Log, Server was eleven full-width boxes (1150 px wide on a
desktop for a one-digit number) with units and the meaning of 0 in the
labels, and Directory's Save sat between its settings and the lists below.

Each tab is now named parts in .fsec, the layout #290 gave a feed's settings,
with its switches, numbers with their unit after the box, Server's Save
sticky and greyed until a change, the tabs in their own row on a phone, the
Proxy tag by the account's name, and the log's explanation moved into each
source tab's tooltip.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RqkrVDqdiQEMWAkNGQ2tYA

web/CHANGELOG.md

@@ -31,6 +31,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- The admin page is laid out as a feed's settings are: each tab in named parts, numbers with their unit beside them, switches for on and off, Server's Save always in reach, and on a phone the tabs in a row of their own so all four fit (#414).
 - An item's title and text take four fifths of the reading pane's width, no longer running to its far edge (#360).
 
 ### Added