feat: Implement database feature for system messages

Adds a database functionality to the PhotoReasoningScreen's system message section.

Key features:
- A "Database" button on the system message card opens a pop-up.
- The first pop-up lists saved system message entries with alternating dark yellow backgrounds.
  - A "New" button allows creating new entries.
  - Each entry has a 3-dot menu with a "Delete" option.
- The second pop-up allows creating or editing entries:
  - Fields for "Title" (placeholder "App/Task") and "Guide" (placeholder "Write a guide...").
  - Text fields have a white background; the pop-up has a dark yellow background.
  - A "Save" button saves the entry.
- Entries are stored in SharedPreferences using JSON serialization.
- Basic validation for empty fields and duplicate titles is included.
- Styling (rounded buttons, pop-up sizes, colors) implemented as per requirements.

Fixes compilation errors by adding kotlinx-serialization dependency and plugin to app/build.gradle.kts.

Author: google-labs-jules[bot]

app/build.gradle.kts

@@ -3,6 +3,7 @@
 plugins {
     id("com.android.application")
     id("org.jetbrains.kotlin.android")
+    id("org.jetbrains.kotlin.plugin.serialization") version "1.9.0"
     id("com.google.android.libraries.mapsplatform.secrets-gradle-plugin")
 }
 
@@ -87,5 +88,6 @@ dependencies {
     debugImplementation("androidx.compose.ui:ui-test-manifest")
 
     implementation("com.google.ai.client.generativeai:generativeai:0.9.0")
+    implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.6.0")
 }