Add Doxygen, clang-format, cppcheck, and Gitea CI

Doxygen:
- Doxyfile: minimal config, HTML output to docs/, no LaTeX
- @file/@brief on all source files, full @param/@return on public API
- docs/ added to .gitignore

clang-format (14, Linux brace style, 4-space, column 100):
- .clang-format added
- Applied to entire codebase; this commit is the canonical baseline
- just format rewrites in-place; just format-check is the CI gate

cppcheck (--enable=warning,style,performance,portability):
- Linker-symbol pointer comparisons in startup.c suppressed with
  inline cppcheck-suppress (false positives, not real bugs)
- just lint runs cppcheck; zero warnings required to pass

Dockerfile gains clang-format, cppcheck, doxygen packages so all
tools run inside the existing container -- host stays clean.

Gitea Actions (.gitea/workflows/ci.yml):
- Four parallel jobs: build, format, lint, docs
- All jobs use the same Dockerfile-based image
- Doxygen job fails on any warning line in output

src/power.c

@@ -1,10 +1,13 @@
+/** @file power.c
+ *  @brief Power management implementation.
+ */
 #include "power.h"
 #include "regs.h"
 #include <nrf52840.h>
 #include <cmsis_gcc.h>
 
 /* P0.02 on XIAO BLE  -  adjust to match your schematic */
-#define BUTTON_PIN  2u
+#define BUTTON_PIN 2u
 
 void power_init(void)
 {
@@ -13,14 +16,13 @@ void power_init(void)
 
     NRF_GPIOTE->CONFIG[0] = (gpiote_config_t){
         .bit = {
-            .MODE     = GPIOTE_MODE_EVENT,
-            .PSEL     = BUTTON_PIN,
-            .PORT     = 0u,
+            .MODE = GPIOTE_MODE_EVENT,
+            .PSEL = BUTTON_PIN,
+            .PORT = 0u,
             .POLARITY = GPIOTE_POL_LOTOHI,
-        }
-    }.reg;
+        }}.reg;
 
-    NRF_GPIOTE->INTENSET = (gpiote_inten_t){ .bit.IN0 = 1u }.reg;
+    NRF_GPIOTE->INTENSET = (gpiote_inten_t){.bit.IN0 = 1u}.reg;
 
     NVIC_EnableIRQ(GPIOTE_IRQn);
 }