feat(search): add advanced search UI with panel, results list, and SearchScreen rewrite

Phase 4: Desktop UI composables for advanced search.
- Rewrite SearchScreen.kt to use AdvancedSearchBarState with TextFieldValue
- Add AdvancedSearchPanel with kind presets, author chips, date range, hashtags, language
- Add SearchResultsList with sticky headers for People/Notes/Articles/Other sections
- Make QueryParser.parseDateToTimestamp and QuerySerializer.timestampToDate public

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/search/QueryParser.kt

@@ -280,7 +280,7 @@ object QueryParser {
         )
     }
 
-    internal fun parseDateToTimestamp(dateStr: String): Long? {
+    fun parseDateToTimestamp(dateStr: String): Long? {
         // ISO 8601 formats: YYYY, YYYY-MM, YYYY-MM-DD
         return try {
             val parts = dateStr.split("-")

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/search/QuerySerializer.kt

@@ -84,7 +84,7 @@ object QuerySerializer {
         return parts.joinToString(" ")
     }
 
-    internal fun timestampToDate(timestamp: Long): String {
+    fun timestampToDate(timestamp: Long): String {
         // Convert unix timestamp to YYYY-MM-DD
         var remaining = timestamp
         var year = 1970