feat: add SD card font support with on-device download and web management

Add a complete SD card font subsystem that enables users to install and
use custom fonts beyond the three built-in families. This combines the
back-end firmware support (#1327) with the font configuration, build
pipeline, CI distribution, and user-facing management UI (#1392).

Core font system:
- Custom .cpfont binary format (v4) with multi-style support (regular,
  bold, italic, bold-italic) packed into a single file per size
- On-demand glyph loading from SD card with two-pass prewarm rendering
  to bulk-read glyphs per page, achieving near-flash performance for
  Latin text (~697ms vs ~681ms) and viable CJK rendering (~32% slower)
- Persistent advance cache for layout measurement without SD I/O
- Overflow ring buffer for glyph cache misses during rendering
- Memory-conscious design: only advance tables kept in RAM; glyph
  bitmaps, kern tables, and ligatures loaded on demand from SD

Font management:
- On-device WiFi download from GitHub Releases with manifest-based
  discovery, install/update detection, and progress UI
- Web interface font upload, listing, and deletion via /fonts page
- Manual SD card copy to /fonts/ or /.fonts/ directories
- Font selection integrated into Settings > Reader > Font Family

Build pipeline:
- Declarative YAML config (sd-fonts.yaml) as single source of truth
  for the 17-family font library (serif, sans, mono, accessibility)
- Python converter (fontconvert_sdcard.py) for TTF/OTF to .cpfont with
  FreeType rasterization, class-based kerning, and ligature extraction
- Parallel build orchestrator with variable font instance extraction
- CI workflow publishing versioned + stable releases to a dedicated
  crosspoint-fonts repository with auto-incrementing revision tags
- Centralized version constants (cpfont_version.py) shared across
  build tooling and CI, with firmware headers as manual sync points

Additional fixes:
- CJK characters no longer get hyphens inserted at line breaks
- Advance table eliminates 30+ second stalls during CJK section
  indexing for paragraphs with >512 unique codepoints

Closes #930

Co-authored-by: Zach Nelson <zach@zdnelson.com>
Co-authored-by: Justin <itsthisjustin@users.noreply.github.com>
Co-authored-by: jpirnay <jens@pirnay.com>
Co-authored-by: mcrosson <kemonine@kemonine.info>

src/activities/settings/FontDownloadActivity.h

@@ -0,0 +1,91 @@
+#pragma once
+
+#include <string>
+#include <vector>
+
+#include "FontInstaller.h"
+#include "SdCardFont.h"
+#include "activities/Activity.h"
+#include "util/ButtonNavigator.h"
+
+// JSON schema version of the fonts.json manifest. The canonical version for
+// the build tooling lives in lib/EpdFont/scripts/cpfont_version.py. This
+// firmware-side copy must be bumped manually when the firmware is updated to
+// support a new manifest schema.
+#define FONTS_MANIFEST_VERSION 1
+
+#ifndef FONT_MANIFEST_URL
+// Manifest + .cpfont assets are published by .github/workflows/release-fonts.yml
+// to the crosspoint-fonts repo under the "sd-fonts-m<META>-b<BIN>" tag. The tag
+// pattern must stay in sync with the workflow; it derives its version numbers
+// from lib/EpdFont/scripts/cpfont_version.py.
+#define FONT_MANIFEST_URL_STRINGIFY_INNER(x) #x
+#define FONT_MANIFEST_URL_STRINGIFY(x) FONT_MANIFEST_URL_STRINGIFY_INNER(x)
+#define FONT_MANIFEST_URL                                                                                           \
+  "https://github.com/crosspoint-reader/crosspoint-fonts/releases/download/sd-fonts-m" FONT_MANIFEST_URL_STRINGIFY( \
+      FONTS_MANIFEST_VERSION) "-b" FONT_MANIFEST_URL_STRINGIFY(CPFONT_VERSION) "/fonts.json"
+#endif
+
+class FontDownloadActivity : public Activity {
+ public:
+  explicit FontDownloadActivity(GfxRenderer& renderer, MappedInputManager& mappedInput);
+
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+  void render(RenderLock&&) override;
+  bool preventAutoSleep() override { return state_ == LOADING_MANIFEST || state_ == DOWNLOADING; }
+  bool skipLoopDelay() override { return true; }
+
+ private:
+  enum State {
+    WIFI_SELECTION,
+    LOADING_MANIFEST,
+    FAMILY_LIST,
+    DOWNLOADING,
+    COMPLETE,
+    ERROR,
+  };
+
+  struct ManifestFile {
+    std::string name;
+    size_t size = 0;
+  };
+
+  struct ManifestFamily {
+    std::string name;
+    std::string description;
+    std::vector<std::string> styles;
+    std::vector<ManifestFile> files;
+    size_t totalSize = 0;
+    bool installed = false;
+    bool hasUpdate = false;
+  };
+
+  State state_ = WIFI_SELECTION;
+  FontInstaller fontInstaller_;
+  ButtonNavigator buttonNavigator_;
+
+  // Manifest data
+  std::string baseUrl_;
+  std::vector<ManifestFamily> families_;
+  int selectedIndex_ = 0;
+
+  // Download progress
+  size_t currentFileIndex_ = 0;
+  size_t currentFileTotal_ = 0;
+  size_t fileProgress_ = 0;
+  size_t fileTotal_ = 0;
+  int downloadingFamilyIndex_ = 0;
+  std::string errorMessage_;
+
+  void onWifiSelectionComplete(bool success);
+  bool fetchAndParseManifest();
+  void downloadFamily(ManifestFamily& family);
+  void downloadAll();
+  bool isDownloadAllSelected() const { return selectedIndex_ == 0 && !families_.empty(); }
+  int familyIndexFromList(int listIndex) const { return listIndex - 1; }
+  int listItemCount() const { return families_.empty() ? 0 : static_cast<int>(families_.size()) + 1; }
+  size_t totalUninstalledSize() const;
+  static std::string formatSize(size_t bytes);
+};