feat: add AppConfig INI-based configuration system

Implements a zero-dependency INI parser for application settings with
[batch], [conversion], and [quality] sections. Includes AppConfig::load(),
write_default(), output_format(), and parsed_extensions() helpers, along
with a documented config.ini example file.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config.ini

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+# photo-converter configuration
+# Lines starting with # or ; are comments.
+# Copy this file and adjust paths before running in batch mode:
+#   photo-converter --batch --config config.ini
+
+[batch]
+# Directory containing input images (RAW or standard formats)
+input_dir        = import
+
+# Directory where converted images are written
+output_dir       = output
+
+# Scan input_dir recursively into subdirectories (true/false)
+recursive        = false
+
+# Comma-separated list of file extensions to process (case-insensitive)
+file_extensions  = arw,cr2,cr3,nef,dng,orf,rw2,raf,pef,jpg,jpeg,png,tif,tiff
+
+[conversion]
+# Film type detection strategy:
+#   auto  – NegativeDetector analyses the histogram and orange mask
+#   c41   – force C-41 colour negative processing
+#   bw    – force B&W negative processing
+film_type        = auto
+
+# Output format:
+#   png16  – 16-bit PNG  (lossless, archival quality)
+#   png8   – 8-bit PNG   (lossless, smaller files)
+#   tiff16 – 16-bit TIFF (lossless, for professional editing)
+#   jpg    – 8-bit JPEG  (lossy, smallest files)
+output_format    = png16
+
+# Output bit depth (8 or 16); ignored when format is jpeg
+output_bit_depth = 16
+
+# Automatically detect and crop the film frame (true/false)
+auto_crop        = true
+
+# Apply unsharp-mask sharpening (true/false)
+sharpen          = true
+
+# Invert negative to positive (true/false)
+# Set to false if input is already a positive (slide/print)
+invert           = true
+
+[quality]
+# JPEG output quality [0-100]; only used when output_format = jpg
+jpeg_quality     = 95
+
+# Unsharp-mask strength [0.0-1.0]
+sharpen_strength = 0.5