Search page improvements (#709)

## Description

Improves how search results are displayed:

- Reorders search categories to Movies → TV Shows → Episodes →
Collections, surfacing the most commonly searched content types at the
top.
- Adds a new "Combined search results" preference in Advanced settings →
Search. When enabled, shows all results in a single list instead of
separate categories.
- When Seerr is active, adds Library/Discover tabs to switch between
local library results and Seerr discovery results.
- Hides empty result categories instead of showing "No results"
placeholders.

### Screenshots

<img width="1935" height="1100" alt="Screenshot_20260116_113656"
src="https://github.com/user-attachments/assets/1e29aa3a-3ba2-4f86-ab61-fed683490474"
/>

<img width="1934" height="1095" alt="image"
src="https://github.com/user-attachments/assets/8b93fa38-d4c9-4e72-a997-caa82b4949f0"
/>

<img width="1921" height="1094" alt="image"
src="https://github.com/user-attachments/assets/82aed1a5-810c-413f-a266-6d78490a1534"
/>

### AI/LLM usage
Code auditing and PR description drafting with Claude Code. Also used
Claude Code for the regex.

app/src/main/proto/WholphinDataStore.proto

@@ -165,6 +165,10 @@ message ScreensaverPreferences{
   bool show_clock = 7;
 }
 
+message SearchPreferences {
+  bool combined_search_results = 1;
+}
+
 message InterfacePreferences {
   ThemeSongVolume play_theme_songs = 1;
   bool remember_selected_tab = 2;
@@ -179,6 +183,7 @@ message InterfacePreferences {
   ScreensaverPreferences screensaver_preference = 11;
   bool enable_media_management = 12;
   bool show_logos = 13;
+  SearchPreferences search_preferences = 14;
 }
 
 message AdvancedPreferences {