fix: webserver stability, memory leaks, epub underlining, flash screen

Webserver:
- Remove MD5 hash computation from file listings (caused EAGAIN errors)
- Implement JSON batching (2KB) with pacing for file listings
- Simplify sendContentSafe() flow control

Memory:
- Cache QR codes on server start instead of regenerating per render
- Optimize WiFi scan: vector deduplication, 20 network limit, early scanDelete()
- Fix 48KB cover buffer leak when navigating Home -> File Transfer

EPUB Reader:
- Fix errant underlining by flushing partWordBuffer before style changes
- Text before styled inline elements (e.g., <a> with CSS underline) no longer
  incorrectly receives the element's styling

Flash Screen:
- Fix version string buffer overflow (30 -> 50 char limit)
- Use half refresh for cleaner display
- Adjust pre_flash.py timing for half refresh completion

scripts/pre_flash.py

@@ -5,7 +5,11 @@ This allows the firmware to display "Flashing firmware..." on the e-ink display
 before the actual flash begins. The e-ink retains this message throughout the
 flash process since it doesn't require power to maintain the display.
 
-Protocol: Sends "FLASH:version\n" where version is read from platformio.ini
+Protocol (Plan A - Simple timing):
+1. Host opens serial port and sends "FLASH:version"
+2. Host keeps port open briefly for device to receive and process
+3. Device displays flash screen when it receives the command
+4. Host proceeds with flash
 """
 
 Import("env")
@@ -15,7 +19,7 @@ from version_utils import get_version
 
 
 def before_upload(source, target, env):
-    """Send FLASH command with version to device before upload begins."""
+    """Send FLASH command to device before uploading firmware."""
     port = env.GetProjectOption("upload_port", None)
 
     if not port:
@@ -29,19 +33,20 @@ def before_upload(source, target, env):
         ]
         port = ports[0] if ports else None
 
-    if port:
-        try:
-            version = get_version(env)
-            ser = serial.Serial(port, 115200, timeout=1)
-            ser.write(f"FLASH:{version}\n".encode())
-            ser.flush()
-            ser.close()
-            time.sleep(0.8)  # Wait for e-ink fast refresh (~500ms) plus margin
-            print(f"[pre_flash] Flash notification sent to {port} (version {version})")
-        except Exception as e:
-            print(f"[pre_flash] Notification skipped: {e}")
-    else:
+    if not port:
         print("[pre_flash] No serial port found, skipping notification")
+        return
+
+    try:
+        version = get_version(env)
+        ser = serial.Serial(port, 115200, timeout=1)
+        ser.write(f"FLASH:{version}\n".encode())
+        ser.flush()
+        time.sleep(4.0)  # Keep port open for device to receive and complete full refresh (~2-3s)
+        ser.close()
+        print(f"[pre_flash] Flash notification sent to {port} (version {version})")
+    except Exception as e:
+        print(f"[pre_flash] Notification skipped: {e}")
 
 
 env.AddPreAction("upload", before_upload)