refactor: improve login form and button styling

- Replace hardcoded credentials with appropriate placeholders
- Add descriptive Finnish placeholders for login inputs
- Style 'Start game' button with primary button class and center alignment
- Improve security by removing exposed default credentials

src/lib/components/Login.svelte

@@ -15,7 +15,7 @@
 			class="input-text"
 			required
 			minlength="3"
-			value="admin"
+			placeholder="käyttäjätunnus"
 		/>
 		<label for="password">Salasana</label>
 		<input
@@ -24,8 +24,8 @@
 			id="password"
 			class="input-text"
 			required
-			value="password"
 			minlength="5"
+			placeholder="salasana"
 		/>
 		<hr class="my-4 border-0" />
 		<button type="submit" class="button button-primary">Sisään</button>

src/routes/gameroom/+page.svelte

@@ -104,7 +104,7 @@
 					name="selected_bonus_cliches"
 					value={JSON.stringify(selected_bonus_cliches)}
 				/>
-				<button type="submit">Aloita peli</button>
+				<button type="submit" class="button button-primary mx-auto">Aloita peli</button>
 			</form>
 		{:else}
 			<button class="button button-disabled mx-auto" disabled>Korjaa valintaa</button>