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]>

scripts/classroom-desktop-layout

@@ -3,7 +3,7 @@
 """Match the classroom's panel to the machine it is on.
 
 Which applications a device offers is decided here, once, and written into
-the three feature lines of the installed layout. The panel is then built
+the feature lines of the installed layout. The panel is then built
 from that layout at the next login.
 
 Each application is its own question, and the shape of the machine is
@@ -15,6 +15,11 @@ never one of them:
 
     Parent Call   PARENT_CALL_ENABLED. Same rule.
 
+    Presentations PRESENTATIONS_ENABLED. Same rule again, and it asks
+                  the machine nothing at all: a presentation needs a
+                  screen, which every device has. Alone among these it
+                  defaults to OFF, because no existing room has it.
+
     the disc      DVD_ENABLED *and* a drive being present. The setting can
     player        take the player away from a machine that has a drive;
                   nothing can put it on a machine that has none, because a
@@ -39,6 +44,7 @@ again at the next login.
 """
 
 import glob
+import json
 import os
 import re
 import sys
@@ -61,10 +67,25 @@ FEATURE_LINES = (
     ("WONDERINK", "WONDERINK_ENABLED"),
     ("DVD", "DVD_ENABLED"),
     ("PARENT_CALL", "PARENT_CALL_ENABLED"),
+    ("PRESENTATIONS", "PRESENTATIONS_ENABLED"),
 )
 
 MARKER = "/* CLASSROOM-FEATURES-BEGIN */"
 
+# WHAT THE MANUAL IS ALLOWED TO KNOW ABOUT THIS DEVICE.
+#
+# This command already decides what the device offers, so it is also the
+# place to write that answer down where the classroom account can read
+# it. A list of setting NAMES, and nothing else: no values, no broker
+# password, no Wi-Fi, no update address, none of the rest of device.conf
+# — which the manual has no use for and the room should not be handed.
+#
+# In /run rather than /etc because it is derived, not configured: it is
+# rewritten from device.conf at every boot by the same service that
+# builds the bar, so it cannot go stale behind a setting somebody
+# changed.
+MANIFEST = "/run/ekklesia-classroom/features.json"
+
 # The kernel names optical drives sr0, sr1, ... regardless of bus. Both
 # views are checked: /dev is what anything opening the drive will use, and
 # /sys/block is the kernel's own list, which cannot be left behind by a
@@ -121,6 +142,32 @@ def rewrite(text, variable, answer):
     )
 
 
+def publish(answers, path=MANIFEST):
+    """Leave the enabled features where classroom-manual can read them.
+
+    Best effort on purpose. A room whose manual shows a page for
+    something it has not got is a small confusion; a panel that failed to
+    build because a directory was read-only is a room with no way of
+    getting anywhere.
+    """
+    enabled = sorted(
+        setting for variable, setting in FEATURE_LINES if answers.get(variable)
+    )
+
+    try:
+        os.makedirs(os.path.dirname(path), exist_ok=True)
+        temporary = f"{path}.new"
+
+        with open(temporary, "w", encoding="utf-8") as handle:
+            json.dump(enabled, handle)
+
+        os.replace(temporary, path)
+        os.chmod(path, 0o644)
+        say(f"the manual will show {', '.join(enabled) or 'no optional pages'}")
+    except OSError as error:
+        say(f"could not write {path}: {error}")
+
+
 def main():
     path = sys.argv[1] if len(sys.argv) > 1 else LAYOUT
 
@@ -137,8 +184,11 @@ def main():
         return 0
 
     after = before
+    answers = wanted(read_device_config())
 
-    for variable, answer in wanted(read_device_config()).items():
+    publish(answers)
+
+    for variable, answer in answers.items():
         after = rewrite(after, variable, answer)
 
     if after == before: