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/printer_service.h

@@ -24,7 +24,7 @@ public:
     void sleep();
     void feed(int lines);
 
-    void setHeat(uint8_t heatTime);
+    void setHeat(uint8_t heatTime, uint8_t heatInterval);
 
     void printBitmap(int w, int h, const uint8_t *bitmap);
     void printText(const String &text);