feat: allow users to specify a custom font (closes #176)

Adds support for loading a custom font from either a remote URL or a
local file path, and applying it to the terminal and input bar.

- Rust: adds `custom_font_path` and `custom_font_family` fields to
  `HikariConfig` with `#[serde(default)]` for backwards compatibility
- TypeScript: extends `HikariConfig` interface and `defaultConfig`;
  exports `applyCustomFont()` which injects an `@import` for CSS
  stylesheet URLs, a `@font-face` rule for direct font file URLs, or a
  base64 data URL `@font-face` for local files via Tauri `readFile`;
  adds `setCustomFont()` to the config store
- Terminal.svelte and InputBar.svelte now use
  `--terminal-font-family` CSS variable (falls back to `monospace`)
- ConfigSidebar.svelte: new "Custom Font" section with URL/path input,
  family name input, Apply + Reset buttons, and inline status feedback
- `+page.svelte`: applies saved font on startup alongside theme/size
- 14 new tests for `applyCustomFont` (all code paths) + 2 for
  `setCustomFont`

src/lib/components/StatusBar.svelte

@@ -111,6 +111,8 @@
     trusted_workspaces: [],
     background_image_path: null,
     background_image_opacity: 0.3,
+    custom_font_path: null,
+    custom_font_family: null,
   });
 
   let streamerModeActive = $state(false);