Storage: view pictures, PNG, JPEG, BMP and GIF (#45)

Enter on a picture shows it: shrunk to fit the screen, or at its own size
with Enter again and the arrows to move. Dithered to the screen's 256
colours; a colour the screen has exactly is left alone, so screenshots are
shown as they are.

The picture is decoded once, straight into the screen's buffer, and kept
there (App::retainsContent): no copy in memory. Decoding runs on the
storage task, so the keys keep working and a 12 megapixel photograph
appears as it comes instead of tripping the watchdog.

PNG, BMP and GIF are read by decoders of our own, host-tested against files
made by Pillow; the PNG one needs 32 KB where the display library's needed
44 KB in one block, which the device often doesn't have. JPEG uses the
library's TJpgDec.

Also corrects two sentences that still gave 16 KB as the editing limit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

README.md

@@ -138,7 +138,8 @@ A copy runs in the background of the card (about 400 KB a second) in short turns
 
 Enter on a file opens it by type; Tab switches to the same file as a hex dump or as text:
 
-- **Text** (`.txt`, `.log`, `.gmi`, `.csv`, and anything that looks like text): only the screen's worth is read from the card, so a file of any size opens at once. Logs open at the end. Up and Down move a line, Left and Right a page, `t` and `b` go to the top and the end, `e` edits it (up to 16 KB, see Notes).
+- **Text** (`.txt`, `.log`, `.gmi`, `.csv`, and anything that looks like text): only the screen's worth is read from the card, so a file of any size opens at once. Logs open at the end. Up and Down move a line, Left and Right a page, `t` and `b` go to the top and the end, `e` edits it, whatever its size (see Notes).
+- **Pictures** (`.png`, `.jpg`, `.bmp`, `.gif`; issue #45): shrunk to fit the screen, or at their own size with Enter, the arrows then moving half a screen at a time; `i` gives the size in pixels. Dithered to the screen's 256 colours, decoded straight into the screen's buffer with no copy in memory, on the storage task so the keys keep working (12 megapixels of JPEG: 7 s). A GIF shows its first picture. A progressive JPEG or an interlaced PNG opens as hex, with the reason.
 - **Captures** (`.pcap`): the packets as the LoRa Scanner lists them; Enter shows one with its Meshtastic header and bytes.
 - **Tracks** (`.gpx`): the number of points, the start, the duration and the distance.
 - **Update Files** (`.ota`): the version, and whether the file would install: it's checked as an install checks it (signature and contents) without writing anything. Enter then installs it.