Add EXIF Editor, consolidate channel key resolution

EXIF Editor (Library → CLI → API → WebUI):
- src/stegasoo/utils.py: read_image_exif(), write_image_exif()
- CLI: stegasoo tools exif [--clear|--set Field=Value]
- API: /api/tools/exif, /api/tools/exif/update, /api/tools/exif/clear
- WebUI: EXIF Editor tab with inline editing, clear all, save/download

Architectural consolidation:
- Moved resolve_channel_key() to src/stegasoo/channel.py (was duplicated in 3 frontends)
- Added get_channel_response_info() for consistent API/WebUI responses
- Frontends now use thin wrappers that translate exceptions

DCT improvements:
- Added will_fit_by_mode() pre-check to WebUI encode (fail fast)
- Suggests LSB mode when DCT capacity exceeded

Dependencies:
- Added piexif>=1.1.0 for EXIF editing

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontends/cli/main.py

@@ -168,37 +168,25 @@ def resolve_channel_key_option(
     """
     Resolve channel key from CLI options.
 
+    Wrapper around library's resolve_channel_key with Click exception handling.
+
     Returns:
         None: Use server-configured key (auto mode)
         "": Public mode (no channel key)
         str: Explicit channel key
     """
-    if no_channel:
-        return ""  # Public mode
+    from stegasoo.channel import resolve_channel_key
 
-    if channel_file:
-        # Load from file
-        path = Path(channel_file)
-        if not path.exists():
-            raise click.ClickException(f"Channel key file not found: {channel_file}")
-        key = path.read_text().strip()
-        if not validate_channel_key(key):
-            raise click.ClickException(f"Invalid channel key format in file: {channel_file}")
-        return key
-
-    if channel:
-        if channel.lower() == "auto":
-            return None  # Use server config
-        # Explicit key provided
-        if not validate_channel_key(channel):
-            raise click.ClickException(
-                "Invalid channel key format. Expected: XXXX-XXXX-XXXX-XXXX-XXXX-XXXX-XXXX-XXXX\n"
-                "Generate a new key with: stegasoo channel generate"
-            )
-        return channel
-
-    # Default: use server-configured key (auto mode)
-    return None
+    try:
+        return resolve_channel_key(
+            value=channel,
+            file_path=channel_file,
+            no_channel=no_channel,
+        )
+    except FileNotFoundError as e:
+        raise click.ClickException(str(e))
+    except ValueError as e:
+        raise click.ClickException(str(e))
 
 
 def format_channel_status_line(quiet: bool = False) -> str | None: