feat: Presentations, for a teacher who downloaded tonight's session

PRESENTATIONS_ENABLED, off by default, independent of everything else. A
teacher downloads a session in ordinary Chrome and it is on the
Presentations screen a few seconds later, ready for the television. No
file manager, no import button, no Impress.

WHY NOTHING LAUNCHES A DOCUMENT APPLICATION. The brief was to use
existing presentation software, so the existing software was tried
first, on Debian 13, before any of this was designed. `soffice --show`
does start a real fullscreen slideshow — and needs XDG_SESSION_TYPE plus
a VCL plugin or it dies with "Failed to open display", shows a splash,
opened a "Tip of the Day: 1/225" dialog over the slides, and leaves the
Impress editor when the show ends. `okular --presentation` opens THREE
windows: the presentation, the ordinary document window behind it, and
an information dialog. zathura is not a presenter. pdfpc is the closest
and is another dependency with a presenter-console model. Every one of
them leaves an application on screen that can open any file on the
machine, and none of them knows there is a library to go back to.

So LibreOffice converts once, headless, at import, with no window on any
screen; poppler renders the pages; the classroom shows the pictures.
Measured: 0.34s for a ten-slide PPTX, 0.03s for its ten thumbnails,
0.10s for one slide at 1920px. That is not a new renderer — it is the
same engine Impress would have used live — and it means Escape is ours,
presenting is instant, and a PDF exported from Google Slides skips the
conversion entirely and keeps the appearance Google gave it.

THE THREE THINGS KEPT APART. The library thumbnail, the starting slide
and the retention clock are independent, because the first slide of a
Bible study session is a generic opening shared by every session and the
one that names it is usually the second. Quick Look chooses the picture;
it does not change where the slideshow starts and it does not count as
having presented anything. mark_presented() is called when a slide is
actually drawn, not when Present is pressed: an intention is not a
presentation, and a viewer that failed to start must not buy another
thirty days.

IMPORTING. The teacher's download is copied, never moved, renamed or
deleted. A file with a .crdownload beside it is still arriving. Wonder
Ink's own folder is not looked at. Two downloads of the same name are
two presentations, the second titled (2) — found in the lab, where a PDF
and a PPTX of one session both came out with the same name because the
import being named was excluded from the taken-titles set BY TITLE, and
a set holds one copy of a repeated string.

Metadata is written saying "importing" before any work, so a reboot
mid-conversion leaves a directory the next startup sweeps and imports
again. A file that cannot be converted gets a card saying so, offering
only Remove, with the teacher's download untouched.

Retention: 180 days never presented, 30 days from the last presentation,
every presentation starting the thirty over. The cleanup never touches a
presentation being shown, never leaves the library directory, and never
looks at Downloads.

The dock order is unchanged: Presentations is appended after the disc
player, so Wonder Ink, the browser and the player keep the places
teachers have been reaching for. There is a test for exactly that.

classroom-wonderink now asks feature() rather than carrying its own copy
of the default, so the panel and the command cannot disagree.

Co-Authored-By: Claude Opus 5 <[email protected]>

config/icons/ekklesia-presentations.svg

@@ -0,0 +1,35 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 22 22" width="22" height="22">
+  <!-- The comment lives inside <svg>, not above it. gdk-pixbuf
+       recognises an SVG by sniffing the first bytes of the file, and a
+       comment before the opening tag pushes it out of that window:
+       every GTK program then reports "couldn't recognize the image file
+       format" while Qt renders it happily. Measured on both of the
+       classroom's other icons, and not learned again here. -->
+  <!-- Presentations.
+
+       Breeze's own view-presentation is a bar chart on a board, which in
+       a church reads as a business meeting rather than as tonight's
+       Bible study. x-office-presentation is a projector screen and comes
+       closest, but it is a filetype mark: it says "a file of this kind"
+       where the panel needs "the place your slides are".
+
+       So: a screen on a stand with a slide showing on it — a title line
+       and two lines of text, which is the shape of every one of these
+       sessions. Same 22-unit grid, same single-path style and the same
+       ColorScheme-Text class as Parent Call, so it recolours with the
+       session and needs no second file for the light theme. -->
+  <defs>
+    <style type="text/css">.ColorScheme-Text { color: #fcfcfc; }</style>
+  </defs>
+  <g class="ColorScheme-Text" style="fill:currentColor;fill-opacity:1;stroke:none">
+    <!-- The screen: an outline, so the slide inside it reads as content
+         rather than the whole mark being one solid block at 22px. -->
+    <path d="M2 3h18a1 1 0 0 1 1 1v10a1 1 0 0 1-1 1H2a1 1 0 0 1-1-1V4a1 1 0 0 1 1-1zm1 2v8h16V5z"/>
+    <!-- The slide on it: a title, and two lines under it. -->
+    <path d="M5 7h7v1.5H5zm0 3h12v1H5zm0 2.2h9v1H5z" style="fill-opacity:0.85"/>
+    <!-- The stand. A presentation is a thing set up in front of a room,
+         and the two feet are what say so at this size. -->
+    <path d="M10.25 16h1.5v3.2l2.9 1.45-.67 1.34-2.98-1.49-2.98 1.49-.67-1.34 2.9-1.45z"/>
+  </g>
+</svg>