feat: comprehensive balance and bug fix pass (#240)

## Summary

- **fix(#148)**: Boss fights now return a fresh HMAC signature in the response; both the manual and auto-boss paths update `signatureReference` from it, ending the signature-mismatch loop that stopped auto-boss after the first fight
- **fix(#145)**: Militia `baseCost` lowered from 100g → 65g, smoothing the peasant→militia jump from 10× to ~6.5×
- **fix(#144)**: `crystal_shard` buffed from `1.65×/1.2×` → `1.9×/1.3×` — now competitive as an epic trinket
- **fix(#142)**: Click-power recipe progression smoothed across zones 13–18 and ceiling raised: z13 1.20→1.22, z15 1.22→1.25, z17 1.25→1.28, z18 1.28→1.30
- **close(#143)**: `elder_bark_shield` (1.2×), `void_fragment_amulet` (1.15×), and `soul_bound_catalyst` (1.2×) are all already at or above their target values from a prior pass

Closes #148
Closes #145
Closes #144
Closes #142

Reviewed-on: #240
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

apps/web/src/context/gameContext.tsx

@@ -1496,11 +1496,20 @@ export const GameProvider = ({
               });
 
               /*
-               * Boss fight modifies server state; clear stale signature so
-               * the next pre-save or auto-save does not send a mismatched one.
+               * Boss fight modifies server state; update signature chain so
+               * the next pre-save or auto-save sends the correct token.
                */
-              signatureReference.current = null;
-              localStorage.removeItem("elysium_save_signature");
+              if (result.signature === undefined) {
+                signatureReference.current = null;
+                localStorage.removeItem("elysium_save_signature");
+              } else {
+                signatureReference.current = result.signature;
+                localStorage.setItem(
+                  "elysium_save_signature",
+                  result.signature,
+                );
+              }
+              lastSaveReference.current = Date.now();
               setAutoBossLastResult({
                 at:       Date.now(),
                 bossName: bossName,
@@ -2177,6 +2186,14 @@ export const GameProvider = ({
         }
         return applyBossResult(previous, bossId, result);
       });
+      if (result.signature === undefined) {
+        signatureReference.current = null;
+        localStorage.removeItem("elysium_save_signature");
+      } else {
+        signatureReference.current = result.signature;
+        localStorage.setItem("elysium_save_signature", result.signature);
+      }
+      lastSaveReference.current = Date.now();
       setBattleResult({ bossName: boss.name, result: result });
     } catch (error_: unknown) {
       const bossErrorMessage