feat(ui): fill the blank message pane with a branded placeholder

An empty right pane said nothing, and multi-select made it a routine
sight. It now carries the wordmark, thread counts that run their query
when clicked, and a sync line that appears only when something needs
attention.

Rendered into the existing web view as a third document shape, so there
is one document path and one set of security rules. The brand palette is
a deliberate exception to deriving colours from the desktop theme, since
a logo is brand rather than chrome; the theme still picks which of the
two sets is used.

Counts refresh when the pane is about to show rather than in the
background: one goes stale the moment a tag is edited, and refreshing one
nobody is looking at is work for nothing. A generation counter discards a
superseded reply, and a late answer cannot repaint over an opened thread.

The helper lines are real links because JavaScript is off in this
profile. The handler is gated on the placeholder actually being
displayed, so the same URL inside a message body is dropped: a stranger's
mail must not drive the thread list, even to run a harmless query.

Three defects found while building, all silent:

- Every CSS percentage was invalid. QString::arg does not collapse "%%"
  into "%", so the document carried "50%%" and the browser dropped each
  declaration holding one, disabling the mask, the glow and both radial
  gradients while still rendering something plausible. Substitution is by
  named token now, which cannot collide with a percent sign.
- A geometry probe endorsed the layout while that was live, because it
  measured only properties without percentages.
- The font test passed against a build with one face missing, since the
  other satisfied both of its checks on its own.

The mockup's light values needed correcting against a real pane: the grid
vanished at a 2% luminance step on white, and the glow subtracts light
there rather than adding it, washing the pane. Strength only, not hue.

1 files changed, 7 insertions, 1 deletions

diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 0f6ec5a..c09ef78 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -1,6 +1,12 @@
 # add_qtmaildir_test(<name>) builds tests/test_<name>.cpp and registers it.
+#
+# resources.qrc is compiled into each test binary for the same reason the app
+# compiles it rather than the static library: a qrc inside a .a registers from
+# a global initialiser the linker then drops. Library code reads :/fonts/ when
+# it builds the placeholder pane, so a test that never links the qrc would
+# exercise only the missing-resource fallback and pass against a broken build.
 function(add_qtmaildir_test name)
-    add_executable(test_${name} test_${name}.cpp)
+    add_executable(test_${name} test_${name}.cpp ${CMAKE_SOURCE_DIR}/src/resources.qrc)
     target_link_libraries(test_${name} PRIVATE qtmaildir_lib Qt6::Test)
     add_test(NAME ${name} COMMAND test_${name})
 endfunction()