fix bugs, harden server, strip-based dithering, update docs

Firmware:
- fix 90° rotation (was transpose)
- fix Adafruit_Thermal constructor (spurious DTR pin arg)
- wire up uploadImage; heatInterval now applied
- PSRAM-free strip dithering: 2-row buffers (~1KB) replace 153KB PSRAM alloc
- consolidate all pins into config.h; BUTTON_PIN → Config::PIN_BUTTON
- constrain contrast/brightness/heat in settings save handler
- uploadImage size param int → size_t

Server:
- canonicalize upload_dir at startup (fixes path traversal guard)
- path traversal guard in serve_image
- replace unwrap in gallery_data with error handling
- IMAGE_WIDTH/IMAGE_HEIGHT named constants

Gallery:
- innerHTML → createElement (XSS-safe)
- encodeURIComponent on image URLs
- replace("_"," ") → regex /_/g

Docs: rewrite README, clarify GUEST_MANUAL placeholders

include/image_processor.h

@@ -7,5 +7,5 @@ class ImageProcessor
 {
 public:
     static void processAndPrint(const CameraService::Frame &frame, PrinterService &printer, const AppSettings &settings);
-    static void uploadImage(const uint8_t *bitmap, int size, const String &url);
+    static void uploadImage(const uint8_t *bitmap, size_t size, const String &url);
 };