feat: add SiteMetadataCache for per-account site metadata storage

SharedPreferences-based cache for GhostSite metadata keyed by account ID.
Supports save/get/getVersion/remove operations with Gson serialization.
Includes Robolectric tests for round-trip, overwrite, multi-account
isolation, and removal.

app/src/main/java/com/swoosh/microblog/data/SiteMetadataCache.kt

@@ -0,0 +1,42 @@
+package com.swoosh.microblog.data
+
+import android.content.Context
+import android.content.SharedPreferences
+import com.google.gson.Gson
+import com.swoosh.microblog.data.model.GhostSite
+
+class SiteMetadataCache(context: Context) {
+
+    private val prefs: SharedPreferences = context.getSharedPreferences(
+        PREFS_NAME, Context.MODE_PRIVATE
+    )
+    private val gson = Gson()
+
+    fun save(accountId: String, site: GhostSite) {
+        val json = gson.toJson(site)
+        prefs.edit().putString(keyForAccount(accountId), json).apply()
+    }
+
+    fun get(accountId: String): GhostSite? {
+        val json = prefs.getString(keyForAccount(accountId), null) ?: return null
+        return try {
+            gson.fromJson(json, GhostSite::class.java)
+        } catch (e: Exception) {
+            null
+        }
+    }
+
+    fun getVersion(accountId: String): String? {
+        return get(accountId)?.version
+    }
+
+    fun remove(accountId: String) {
+        prefs.edit().remove(keyForAccount(accountId)).apply()
+    }
+
+    private fun keyForAccount(accountId: String): String = "site_$accountId"
+
+    companion object {
+        const val PREFS_NAME = "site_metadata_cache"
+    }
+}

app/src/test/java/com/swoosh/microblog/data/SiteMetadataCacheTest.kt

@@ -0,0 +1,161 @@
+package com.swoosh.microblog.data
+
+import android.app.Application
+import androidx.test.core.app.ApplicationProvider
+import com.swoosh.microblog.data.model.GhostSite
+import org.junit.Assert.*
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.annotation.Config
+
+@RunWith(RobolectricTestRunner::class)
+@Config(application = Application::class)
+class SiteMetadataCacheTest {
+
+    private lateinit var cache: SiteMetadataCache
+
+    @Before
+    fun setUp() {
+        val context = ApplicationProvider.getApplicationContext<Application>()
+        cache = SiteMetadataCache(context)
+    }
+
+    @Test
+    fun `save and get round trip`() {
+        val site = GhostSite(
+            title = "My Blog",
+            description = "A test blog",
+            logo = "https://example.com/logo.png",
+            icon = "https://example.com/icon.png",
+            accent_color = "#ff1a75",
+            url = "https://example.com/",
+            version = "5.82.0",
+            locale = "en"
+        )
+
+        cache.save("account-1", site)
+        val retrieved = cache.get("account-1")
+
+        assertNotNull(retrieved)
+        assertEquals("My Blog", retrieved!!.title)
+        assertEquals("A test blog", retrieved.description)
+        assertEquals("https://example.com/logo.png", retrieved.logo)
+        assertEquals("https://example.com/icon.png", retrieved.icon)
+        assertEquals("#ff1a75", retrieved.accent_color)
+        assertEquals("https://example.com/", retrieved.url)
+        assertEquals("5.82.0", retrieved.version)
+        assertEquals("en", retrieved.locale)
+    }
+
+    @Test
+    fun `get returns null for unknown account`() {
+        val result = cache.get("nonexistent-account")
+        assertNull(result)
+    }
+
+    @Test
+    fun `getVersion returns version string`() {
+        val site = GhostSite(
+            title = "Blog",
+            description = null,
+            logo = null,
+            icon = null,
+            accent_color = null,
+            url = null,
+            version = "5.82.0",
+            locale = null
+        )
+
+        cache.save("account-2", site)
+        assertEquals("5.82.0", cache.getVersion("account-2"))
+    }
+
+    @Test
+    fun `getVersion returns null for unknown account`() {
+        assertNull(cache.getVersion("nonexistent"))
+    }
+
+    @Test
+    fun `save overwrites existing data`() {
+        val site1 = GhostSite(
+            title = "Old Title",
+            description = null,
+            logo = null,
+            icon = null,
+            accent_color = null,
+            url = null,
+            version = "5.0.0",
+            locale = null
+        )
+        val site2 = GhostSite(
+            title = "New Title",
+            description = "Updated",
+            logo = null,
+            icon = null,
+            accent_color = null,
+            url = null,
+            version = "5.82.0",
+            locale = null
+        )
+
+        cache.save("account-3", site1)
+        cache.save("account-3", site2)
+        val retrieved = cache.get("account-3")
+
+        assertEquals("New Title", retrieved?.title)
+        assertEquals("Updated", retrieved?.description)
+        assertEquals("5.82.0", retrieved?.version)
+    }
+
+    @Test
+    fun `different accounts have independent data`() {
+        val site1 = GhostSite(
+            title = "Blog One",
+            description = null,
+            logo = null,
+            icon = null,
+            accent_color = null,
+            url = null,
+            version = "5.0.0",
+            locale = null
+        )
+        val site2 = GhostSite(
+            title = "Blog Two",
+            description = null,
+            logo = null,
+            icon = null,
+            accent_color = null,
+            url = null,
+            version = "4.48.0",
+            locale = null
+        )
+
+        cache.save("account-a", site1)
+        cache.save("account-b", site2)
+
+        assertEquals("Blog One", cache.get("account-a")?.title)
+        assertEquals("Blog Two", cache.get("account-b")?.title)
+    }
+
+    @Test
+    fun `remove deletes cached data`() {
+        val site = GhostSite(
+            title = "To Remove",
+            description = null,
+            logo = null,
+            icon = null,
+            accent_color = null,
+            url = null,
+            version = "5.0.0",
+            locale = null
+        )
+
+        cache.save("account-remove", site)
+        assertNotNull(cache.get("account-remove"))
+
+        cache.remove("account-remove")
+        assertNull(cache.get("account-remove"))
+    }
+}