Fix export filename extension case handling (fixes #19050)

[sjmudd]

Export filename extension case (fixes #19050)

Problem

Exported filenames always got a hard-coded lowercase extension (.jpg,
.png, ...) appended by the storage module, regardless of what the
filename pattern already produced. Two consequences:

• No way to control the case of the export extension (e.g. get .JPG
  to match camera-style naming like IMG_1234.CR3).
• Embedding an extension explicitly in the pattern (e.g.
  $(FILE_NAME).JPG) produced a duplicated extension:
  IMG_1234.JPG.jpg.

There was also no variable exposing the export format's extension.
$(FILE_EXTENSION) reflects the source file's extension only, so
$(FILE_NAME).$(FILE_EXTENSION^^) does not give the export extension
at all — for a CR3 processed to JPEG it expands to IMG_1234.CR3.

Changes

1. Don't double-append a matching extension
(common/utility.c/.h, imageio/storage/{disk,gallery,latex}.c)

Added dt_util_str_ends_with_extension(), a case-insensitive
"does this filename already end in .ext" check. Each storage
module now skips appending its implicit extension when the expanded
pattern already ends with it (in any case). Patterns without an
extension behave exactly as before.

In disk.c, the "generate unique filename on conflict" handling
reuses the same insertion point to write _01, _02, ... suffixes
before the extension on repeated exports to the same target. That
insertion point is now computed consistently whether the extension
was already present in the pattern or just appended, so conflict
suffixes land in the right place (name_01.jpg) instead of after an
already-present extension (name.JPG_01.jpg).

2. New $(EXPORT_EXTENSION) variable
(common/variables.c/.h, gui/gtkentry.c,
imageio/storage/{disk,gallery,latex,piwigo}.c)

Exposes the extension of the export format actually being used
(format->extension(fdata)), as opposed to $(FILE_EXTENSION)
which is the source file's extension. Works with the existing
^/^^/,/,, case operators like any other variable.

Result

$(FILE_NAME).$(EXPORT_EXTENSION^^)

now expands correctly and dynamically for whatever format is being
exported to, e.g. IMG_1234.CR3 → IMG_1234.JPG when exporting to
JPEG, with no duplicated extension.

Scope

disk (file export) and the GUI's EXPORT_EXTENSION availability
cover the reported case. gallery and latex storage got the same
double-append fix for consistency (same underlying pattern, same
bug). piwigo's pattern-driven path doesn't append an extension
itself, so it only gained $(EXPORT_EXTENSION) support, not the
double-append fix (nothing to fix there). email storage builds its
filename internally, not from a user pattern, so it's unaffected.

Testing

All changed files compile cleanly under the project's CI flags
(gcc-16, -Werror -Wfatal-errors, Ubuntu 26.04 toolchain).
Verified end-to-end with darktable-cli exporting a real Canon CR3
sample:

• $(FILE_NAME).JPG (JPEG export) → single correctly-cased
  extension, no duplication.
• Default pattern, no explicit extension (JPEG export) → unchanged
  .jpg behavior, no regression.
• $(FILE_NAME).$(EXPORT_EXTENSION^^) (JPEG export) →
  IMG_1234.JPG.
• $(FILE_NAME).$(EXPORT_EXTENSION^^) (PNG export, same source
  file) → IMG_1234.PNG — confirms the variable tracks the actual
  export format rather than being hard-coded to one.
• $(FILE_NAME).$(EXPORT_EXTENSION,,) (JPEG export) →
  IMG_1234.jpg — lowercase operator works as expected.
• $(FILE_NAME).JPG (JPEG export) exported three times to the same
  location with "generate unique filename" conflict handling →
  IMG_1234.JPG, IMG_1234_01.jpg, IMG_1234_02.jpg.

Automated tests added:

• src/tests/variables.c gained a test_export_extension case
  covering $(EXPORT_EXTENSION) with all four case operators, and
  the $(FILE_NAME).$(EXPORT_EXTENSION^^) combination, including a
  case confirming $(FILE_EXTENSION) and $(EXPORT_EXTENSION) stay
  independent (source vs. export format extension).
• New cmocka unit test src/tests/unittests/util/test_utility.c
  for dt_util_str_ends_with_extension() and the extracted
  dt_util_str_extension_offset() helper, including the exact
  matching-extension-plus-conflict-suffix case that caused the
  disk.c bug above.

[da-phil]

I'm wondering why this is needed only for windows, does the add_cmocka_test(... LINK_LIBRARIES lib_darktable) not work properly on windows?

[sjmudd]

My understanding is Windows has no RPATH, so the DLL must be copied next to the test binary manually or it won't be found at runtime.

[Donatzsky]

What happens in these case?

$(FILE_NAME).JPG (PNG export).
$(FILE_NAME).JPG (PNG export) exported three times.