Teach the file manager to send and to shrink

Right-click a file to send it to your phone, or a video to make it
smaller. macOS has both behind the Share sheet and Quick Actions;
Windows has "Send to"; a stock Linux file manager has neither, and the
usual answer for the second one is a web uploader or an ffmpeg
incantation looked up again every time.

Neither adds machinery. Sending reuses panama-kdeconnect, the same
helper the Home & Phone page and quick settings already drive, so
there is one way to talk to a phone rather than two. The entry appears
only when a phone is actually reachable: an item that is present and
fails is worse than one that is absent, because the absence explains
itself.

The transcoder's two rules are both about not losing work. It never
writes to its input, and it never writes over an earlier output -- a
second run produces -2 rather than eating the first result. Verified
against a real encode: 1920x1080 became 854x480, with an even width
because H.264 rejects an odd one at the very end of a long encode,
which is the worst possible moment to find out.

Menus decide by mime type rather than extension, act on one file at a
time, and refuse anything that is not a local path. nautilus-python
turned out to be declared already; it now says it carries Panama's own
extensions too.

README.md

@@ -114,7 +114,7 @@ docs/           Settings reference, and the design specs behind the work
 
 ## Tests
 
-146 of them, under `tests/`. Run the lot, or a subset by pattern:
+147 of them, under `tests/`. Run the lot, or a subset by pattern:
 
 ```sh
 panama test           # everything