Fix file:// URI handling for desktop-opened comics

[JoshCLWren]

Problem

The macOS/Linux file-association path handling in cdisplayagain.py incorrectly slices file:// arguments:

if raw.startswith("file://"):
    raw = raw[7]

raw[7] returns one character, not the path after the URI prefix. A file opened through an association that supplies a file:// URI can therefore become a one-character path and fail with “File not found”.

Fix

• Parse file URIs with urllib.parse.urlparse/unquote, including percent-encoded spaces and non-ASCII paths.
• Preserve normal filesystem paths and command-line paths unchanged.
• Handle file:///absolute/path, rather than assuming a simple string slice.

Tests

Add tests for:

• Plain absolute path.
• file:///tmp/My%20Comic.cbz.
• A path containing Unicode characters.
• A non-file URI or malformed URI with an actionable error.

The regression test must fail against the current raw[7] implementation.

Done criteria

• A CBZ/CBR opened through a desktop association resolves to the complete filesystem path.
• Existing direct CLI path handling remains unchanged.
• Lint and the full test suite pass.