[wonjoonSeol]

Fixes #626.

Problem

When a comic opened from the file system can't be read, the error is shown and after that the previous and next comic shortcuts (default Ctrl+Left/Right, shortcuts_manager.cpp#L41-L42) do nothing, so the reader can't move on to the next file in the folder.

Screenshots

Linux build run offscreen with generated sample pages, taken on my fork, which carries the first commit of this PR. "Upstream" runs the same steps on develop at 268288b.

A folder with Comic 1.pdf, an unreadable Comic 2.cbz and Comic 3.pdf. Going from Comic 1 to the next comic shows:

After closing the error:

	Previous / next comic buttons
Upstream
This PR

With this PR, the next comic action (Ctrl+Right) then opens Comic 3:

Cause

A failed open resets the viewer, and the reset ends in MainWindowViewer::processReset:

• Render::errorOpening() is connected to Viewer::resetContent (viewer.cpp#L289).
• Render::errorOpening(QString), which carries "Format not supported" (comic.cpp#L610, #L626), goes to Viewer::showMessageErrorOpening(QString) (viewer.cpp#L291), which calls resetContent after the message (viewer.cpp#L379).

resetContent emits reset (viewer.cpp#L1821), which is connected to processReset (main_window_viewer.cpp#L248). For comics not opened from the library, processReset always disables the previous and next comic actions (main_window_viewer.cpp#L1340-L1345), although the neighbouring files were already found when the comic was opened (openComic calls getSiblingComics, #L1023 and #L1685-L1692).

Change

• processReset now sets the previous and next comic actions from the neighbouring comics, with the code MainWindowViewer::doubleMangaPageSwitch already had (main_window_viewer.cpp#L1522-L1530). That function only works out those two actions, so it is renamed to refreshPrevNextActions and its two callers use the new name (#L183, #L519). For comics opened from the library the reset now works the actions out from the sibling list, the same way open does (#L971-L972), instead of leaving them as they were. The offscreen run below gives the same result as upstream there.
• The third way into processReset is opening a recent file that no longer exists (main_window_viewer.cpp#L920-L923). There the neighbours still belong to the comic that was open before, while the view shows "Press 'O' to open comic.", so the PR clears previousComicPath and nextComicPath first and the actions stay disabled as before. In library mode this path behaves as before.

Relation to #614

#614 replaces siblingComics.indexOf(currentComicDB) in doubleMangaPageSwitch with currentSiblingIndex, and this PR renames that function. The two don't conflict: git merge-tree of this branch with #614's head (e0711b3) merges cleanly, and refreshPrevNextActions keeps the currentSiblingIndex version. If #614 lands first and you'd rather have this rebased onto it, I'm happy to do that.

Testing

• Builds on Linux (WSL Ubuntu, Qt 6.10.2, -DPDF_BACKEND=poppler).
• CI on my fork, with the same build.yml plus one line adding workflow_dispatch so it can be started by hand: run 40144752327. Every job passed: Code Format Validation, Linux (Qt6 + libarchive), Linux (Qt6 + 7zip), macOS (Qt6 Universal), Windows x64 (Qt6), Windows ARM64 (Qt6), Docker amd64 and arm64 images, and Source Tarball. The Linux, macOS and Windows x64 jobs also run ctest.
• No automated test: nothing in tests/ sets up MainWindowViewer.
• Offscreen run of the Linux build with a small harness (not part of this PR) that drives MainWindowViewer and reads both actions and their toolbar buttons. The folder holds Comic 1.pdf, an unreadable Comic 2.cbz, Comic 3.pdf, Comic 4.pdf and an unreadable Comic 5.cbz. "Upstream" is the same run on develop at 268288b.

Step	Upstream	This PR
Comic 1, next comic, close "Format not supported"	both disabled	both enabled
Next comic again	nothing happens	Comic 3 opens
From Comic 3, previous comic (error), previous comic again	(not reached)	Comic 1 opens
Manga mode, Comic 4, next comic (Comic 5.cbz is the last file and fails)	both disabled	only previous (right in manga mode) enabled, and it opens Comic 4
Comic 3 open, then a recent file that was deleted	both disabled	both disabled
Library mode, list [Comic 2.cbz, Comic 4.pdf], Comic 2.cbz fails	only next enabled	only next enabled
Library mode, then a recent file that was deleted	only next enabled	only next enabled

The library rows set isClient and the sibling list directly in the harness, since the real path needs YACReaderLibrary running. The folder neighbours left over from the earlier rows give both directions there, so these rows show that the reset reads the library list. Two mutations fail the run as expected: reading the folder neighbours in library mode breaks the library rows, and dropping the line that clears the paths in openComicFromRecentAction breaks the deleted recent file row.

Manual checks still to do:

• A corrupt .cbz between two good comics in a folder: open the first, Ctrl+Right, close the error, Ctrl+Right opens the third.
• The same in manga mode (the directions swap).
• From YACReaderLibrary, with an unreadable comic between two good ones.
• Open recent with a file that no longer exists: "Comic not found", and both actions stay disabled.

[luisangelsm]

Thanks