Add prompt to make Stirling your default PDF app (#4890)

# Description of Changes
- Adds a reusable banner component/system to the core app
- Adds banner at the top of the desktop app if Stirling isn't your
default PDF editor, with a button to make it your default
- Adds a permanent button in the settings to do it manually (in case
you've dismissed the banner)
- Simplifies the file loading logic to fix a bug where the input file
could be duplicated occasionally. Now, the TS just receives files from
one buffer, regardless of how they've been passed to the app in Rust.

## Caveats
I've only been able to get the setting of default apps working properly
on Mac. The Windows build isn't signed (yet) so we can't use the proper
API for it, so currently it just sends you to the Settings UI. I've also
not been able to test it on Linux at all.

frontend/src/desktop/services/defaultAppService.ts

@@ -0,0 +1,70 @@
+import { invoke } from '@tauri-apps/api/core';
+
+/**
+ * Service for managing default PDF handler settings
+ * Note: Uses localStorage for machine-specific preferences (not synced to server)
+ */
+export const defaultAppService = {
+  /**
+   * Check if Stirling PDF is the default PDF handler
+   */
+  async isDefaultPdfHandler(): Promise<boolean> {
+    try {
+      const result = await invoke<boolean>('is_default_pdf_handler');
+      return result;
+    } catch (error) {
+      console.error('[DefaultApp] Failed to check default handler:', error);
+      return false;
+    }
+  },
+
+  /**
+   * Set or prompt to set Stirling PDF as default PDF handler
+   * Returns a status string indicating what happened
+   */
+  async setAsDefaultPdfHandler(): Promise<'set_successfully' | 'opened_settings' | 'error'> {
+    try {
+      const result = await invoke<string>('set_as_default_pdf_handler');
+      return result as 'set_successfully' | 'opened_settings';
+    } catch (error) {
+      console.error('[DefaultApp] Failed to set default handler:', error);
+      return 'error';
+    }
+  },
+
+  /**
+   * Check if user has dismissed the default app prompt (machine-specific)
+   */
+  hasUserDismissedPrompt(): boolean {
+    try {
+      const dismissed = localStorage.getItem('stirlingpdf_default_app_prompt_dismissed');
+      return dismissed === 'true';
+    } catch {
+      return false;
+    }
+  },
+
+  /**
+   * Mark that user has dismissed the default app prompt (machine-specific)
+   */
+  setPromptDismissed(dismissed: boolean): void {
+    try {
+      localStorage.setItem('stirlingpdf_default_app_prompt_dismissed', dismissed ? 'true' : 'false');
+    } catch (error) {
+      console.error('[DefaultApp] Failed to save prompt preference:', error);
+    }
+  },
+
+  /**
+   * Check if we should show the default app prompt
+   * Returns true if: user hasn't dismissed it AND app is not default handler
+   */
+  async shouldShowPrompt(): Promise<boolean> {
+    if (this.hasUserDismissedPrompt()) {
+      return false;
+    }
+
+    const isDefault = await this.isDefaultPdfHandler();
+    return !isDefault;
+  },
+};