feat: add achievements system, hint overlay, and progress page polish

Achievements system:
- Add src/lib/server/milestones.ts with full achievement definitions and
  calculation logic (16 achievements: streaks, book set completions,
  community milestones like Overachiever/Procrastinator/Outlier, and fun
  ones like Prodigal Son, Extra Credit, Is This A Joke To You?)
- Wire calculateMilestones() into the progress page server load
- Replace the old ad-hoc milestone cards with a proper achievements grid
  (3/4 col, uniform min-height cards, larger text)
- Change "With God, All Things Are Possible" from "every game solved in 1"
  to "solve in 1 guess for each of the 66 books at least once"

Game page hint overlay:
- After a correct testament/section/first-letter match, display a subtle
  text hint below the verse prompt (e.g. "It is in the Old Testament.")
- Hints fade in 2.8s after a guess (after the row flip animation)
- Hints are only shown to new players (fewer than 3 tracked wins) to
  avoid being patronising to experienced players

Progress page:
- Hide Skill Growth chart with {#if false && showChart} pending rework
- Fix book tier colour scheme: explored=blue, mastered=purple, perfect=emerald
  (was amber/emerald — now consistent across grid, legend, and stat cards)
- Simplify GuessesTable row colour: remove proximity gradient, use flat red
  for wrong guesses
- Add "Come back tomorrow!" encouragement text in CountdownTimer for new
  players (fewer than 3 wins)
- Fix GamePrompt text colour to always be gray-100

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

src/lib/components/CountdownTimer.svelte

@@ -3,6 +3,7 @@
 
 	let timeUntilNext = $state("");
 	let newVerseReady = $state(false);
+	let showEncouragement = $state(false);
 	let intervalId: number | null = null;
 	let targetTime = 0;
 
@@ -41,6 +42,13 @@
 		initTarget();
 		updateTimer();
 		intervalId = window.setInterval(updateTimer, 1000);
+
+		const winCount = Object.keys(localStorage).filter(
+			(k) =>
+				k.startsWith("bibdle-win-tracked-") &&
+				localStorage.getItem(k) === "true",
+		).length;
+		showEncouragement = winCount < 3;
 	});
 
 	onDestroy(() => {
@@ -77,6 +85,13 @@
 			>
 				{timeUntilNext}
 			</p>
+			{#if showEncouragement}
+				<p
+					class="text-xs text-center uppercase tracking-[0.2em] text-gray-500 dark:text-gray-400 font-bold px-4 mt-3"
+				>
+					Come back tomorrow for a new verse!
+				</p>
+			{/if}
 		{/if}
 	</div>
 </div>