v2.7.5: Logbook Web UI menu entry, per-day CSV delete, remove screenshot feature

Adds a "Logbook / WebUI" entry directly in Menu > System, showing the
device's IP and usage instructions (previously only reachable via a
hidden "?" button in Logbook Files). The web export page now supports
downloading and deleting individual daily logbook CSV files, in
addition to the existing merged CSV download.

Fixes a task watchdog reset that could reboot the device mid-download
when exporting the logbook via the built-in web server: the CSV send
loop never yielded the CPU, starving the idle task on larger logbooks.
The export loop now yields periodically (delay(1) every 10 lines and
after each file).

Removes the screenshot feature (camera button, on-device BMP storage,
web-based screenshot browser). Screenshots always came out solid
black - diagnosis showed the SPI readback from the display returns all
zeros regardless of SPI_READ_FREQUENCY, indicating TFT_MISO isn't
wired to the display on this CYD board revision. This is a hardware
limitation, not fixable in software, so the feature has been removed
rather than shipped broken.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/i18n_en.h

@@ -153,9 +153,6 @@ static const char* const I18N_EN[] = {
     "Level flight",
     "Squawk: ",
 
-    "Saved: ",
-    "Screenshot failed",
-
     "Units",
     "Auto (by location)",
     "Metric (km, m, km/h)",
@@ -199,6 +196,11 @@ static const char* const I18N_EN[] = {
     "Skip name",
     "Presets full - remove one first",
     "When adding a preset you can give it a name (e.g. \"Home\"), which is then shown in the overview. Tap the nearest-airport ticker to add it directly as a new preset with its name.",
+
+    "Logbook / WebUI",
+    "Logbook / WebUI",
+    "This web page lets you view, download and individually delete the flight logbook right from a browser. While connected to the same WiFi network, open a browser and go to:",
+    "No WiFi connection - the page is only reachable while connected to a network.",
 };
 
 static_assert(sizeof(I18N_EN) / sizeof(I18N_EN[0]) == (size_t)StringId::COUNT,