cosmetic refactor (#3628)

## Description:

The motivation is to have a single "cosmetic-button" element, so we can
abstract out the cosmetic types. This will make it much easier to add
new cosmetic types in the future.

Unifies PatternButton and FlagButton into a single CosmeticButton
component. Extracts a resolveCosmetics() function that flattens patterns
× color palettes + flags into a ResolvedCosmetic[] with relationship
status pre-computed, replacing duplicated resolution logic across four
callers.

* New CosmeticButton — renders patterns or flags based on
ResolvedCosmetic.type
* New resolveCosmetics() — centralizes ownership/purchase/blocked
resolution
* Extracted PatternPreview — canvas rendering split into its own module
* Added type: "pattern" | "flag" discriminator to Zod cosmetic schemas
* Deleted FlagButton.ts and PatternButton.ts
* Added 320-line test suite for resolveCosmetics


## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

evan

tests/Privilege.test.ts

@@ -39,6 +39,7 @@ const flagCosmetics = {
   colorPalettes: {},
   flags: {
     cool_flag: {
+      type: "flag" as const,
       name: "cool_flag",
       url: "https://example.com/cool.png",
       affiliateCode: null,