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>

pyproject.toml

@@ -51,7 +51,8 @@ dct = [
 ]
 cli = [
     "click>=8.0.0",
-    "qrcode>=7.30"
+    "qrcode>=7.30",
+    "piexif>=1.1.0",
 ]
 compression = [
     "lz4>=4.0.0",
@@ -61,6 +62,7 @@ web = [
     "gunicorn>=21.0.0",
     "qrcode>=7.3.0",
     "pyzbar>=0.1.9",
+    "piexif>=1.1.0",
     # Include DCT support for web UI
     "numpy>=2.0.0",
     "scipy>=1.10.0",