Fix CLI encode format detection and jpegtran -trim bug

CLI encode:
- Auto-detect output format from extension (.jpg → DCT mode, .png → LSB)
- Default to JPEG output for JPEG carriers (preserves DCT benefits)
- Pass embed_mode and dct_output_format to encode function

jpegtran fix (critical for rotation fallback):
- Remove -trim flag which was dropping edge blocks and destroying stego data
- Remove -perfect flag which fails on non-MCU-aligned images
- Plain jpegtran without flags works correctly for lossless rotation

This enables: encode → external rotation → decode to work correctly.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontends/web/app.py

@@ -2136,7 +2136,7 @@ def api_tools_rotate():
                 output_path = tempfile.mktemp(suffix=".jpg")
                 try:
                     result = subprocess.run(
-                        ["jpegtran", "-rotate", str(rotation), "-copy", "all", "-trim",
+                        ["jpegtran", "-rotate", str(rotation), "-copy", "all",
                          "-outfile", output_path, input_path],
                         capture_output=True, timeout=30
                     )
@@ -2158,7 +2158,7 @@ def api_tools_rotate():
                 output_path = tempfile.mktemp(suffix=".jpg")
                 try:
                     result = subprocess.run(
-                        ["jpegtran", "-flip", "horizontal", "-copy", "all", "-trim",
+                        ["jpegtran", "-flip", "horizontal", "-copy", "all",
                          "-outfile", output_path, input_path],
                         capture_output=True, timeout=30
                     )
@@ -2180,7 +2180,7 @@ def api_tools_rotate():
                 output_path = tempfile.mktemp(suffix=".jpg")
                 try:
                     result = subprocess.run(
-                        ["jpegtran", "-flip", "vertical", "-copy", "all", "-trim",
+                        ["jpegtran", "-flip", "vertical", "-copy", "all",
                          "-outfile", output_path, input_path],
                         capture_output=True, timeout=30
                     )

src/stegasoo/cli.py

@@ -241,8 +241,20 @@ def encode(
     with open(carrier, "rb") as f:
         carrier_data = f.read()
 
-    # Determine output path
+    # Determine output path and format
-    output = output or f"{Path(carrier).stem}_encoded.png"
+    # Default to JPEG for JPEG carriers (preserves DCT mode benefits)
+    carrier_ext = Path(carrier).suffix.lower()
+    if not output:
+        if carrier_ext in ('.jpg', '.jpeg'):
+            output = f"{Path(carrier).stem}_encoded.jpg"
+        else:
+            output = f"{Path(carrier).stem}_encoded.png"
+
+    # Detect output format from extension
+    output_ext = Path(output).suffix.lower()
+    use_dct = output_ext in ('.jpg', '.jpeg')
+
+    from .steganography import EMBED_MODE_DCT, EMBED_MODE_LSB
 
     try:
         if file_payload:
@@ -253,6 +265,8 @@ def encode(
                 carrier_image=carrier_data,
                 passphrase=passphrase,
                 pin=pin,
+                embed_mode=EMBED_MODE_DCT if use_dct else EMBED_MODE_LSB,
+                dct_output_format="jpeg" if use_dct else "png",
             )
         else:
             # Encode message
@@ -262,6 +276,8 @@ def encode(
                 carrier_image=carrier_data,
                 passphrase=passphrase,
                 pin=pin,
+                embed_mode=EMBED_MODE_DCT if use_dct else EMBED_MODE_LSB,
+                dct_output_format="jpeg" if use_dct else "png",
             )
 
         # Write output

src/stegasoo/dct_steganography.py

@@ -1252,20 +1252,12 @@ def _jpegtran_rotate(image_data: bytes, rotation: int) -> bytes:
     output_path = tempfile.mktemp(suffix=".jpg")
 
     try:
-        # jpegtran -rotate 90|180|270 -copy all -perfect
+        # jpegtran -rotate 90|180|270 -copy all
         # -copy all: preserve all metadata
-        # -perfect: fail if there are non-transformable edge blocks (rare)
+        # NOTE: Don't use -trim as it drops edge blocks and destroys stego data
+        # NOTE: Don't use -perfect as it fails on images with non-MCU-aligned edges
         result = subprocess.run(
-            ["jpegtran", "-rotate", str(rotation), "-copy", "all", "-perfect",
+            ["jpegtran", "-rotate", str(rotation), "-copy", "all",
-             "-outfile", output_path, input_path],
-            capture_output=True,
-            timeout=30
-        )
-
-        # If -perfect fails (edge blocks), retry without it
-        if result.returncode != 0:
-            result = subprocess.run(
-                ["jpegtran", "-rotate", str(rotation), "-copy", "all", "-trim",
              "-outfile", output_path, input_path],
             capture_output=True,
             timeout=30