fix: SnyggSelector crash on unknown selectors and copy-paste serializationId

SnyggSelector.from() used entries.first() which throws
NoSuchElementException on unrecognized selector strings from imported
theme files. Changed to firstOrNull() with NONE fallback so malformed
themes degrade gracefully instead of crashing the IME.

SnyggContentScaleValue.serializationId was "textAlign" (copy-pasted
from SnyggTextAlignValue). Corrected to "contentScale".

Author: SysAdminDoc

lib/snygg/src/main/kotlin/org/florisboard/lib/snygg/SnyggRule.kt

@@ -542,7 +542,7 @@ enum class SnyggSelector(val id: String) {
         internal fun from(str: String): SnyggSelector {
             if (str.isNotEmpty()) {
                 val selector = str.substring(1)
-                return entries.first { it.id == selector }
+                return entries.firstOrNull { it.id == selector } ?: NONE
             }
             return NONE
         }

lib/snygg/src/main/kotlin/org/florisboard/lib/snygg/value/SnyggUriValue.kt

@@ -54,7 +54,7 @@ data class SnyggUriValue(val uri: String) : SnyggValue {
 
 data class SnyggContentScaleValue(val contentScale: ContentScale) : SnyggValue {
     companion object : SnyggEnumLikeValueEncoder<ContentScale>(
-        serializationId = "textAlign",
+        serializationId = "contentScale",
         serializationMapping = mapOf(
             "crop" to ContentScale.Crop,
             "fill-bounds" to ContentScale.FillBounds,