refactor(ui): rename "By substance" tab to "Substance Log"

Update the nav label, section heading, and the README feature name.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: sparkly-quasar

README.md

@@ -58,7 +58,7 @@ anywhere.
   covers substances you add yourself once they're classified.
 - **Substances** — catalogue substances, assign interaction classes (common ones
   are auto-classified), keep your own dose notes.
-- **By substance** — every dose grouped by substance, so you can see your history
+- **Substance Log** — every dose grouped by substance, so you can see your history
   and typical dosages at a glance.
 
 ## Architecture

src/routes/+page.svelte

@@ -1044,7 +1044,7 @@
         <button class:active={tab === "journal"} onclick={() => goTab("journal")}>Journal</button>
         <button class:active={tab === "companion"} onclick={() => goTab("companion")}>Companion</button>
         <button class:active={tab === "substances"} onclick={() => goTab("substances")}>Substances</button>
-        <button class:active={tab === "bysub"} onclick={() => goTab("bysub")}>By substance</button>
+        <button class:active={tab === "bysub"} onclick={() => goTab("bysub")}>Substance Log</button>
         <button class:active={tab === "data"} onclick={() => goTab("data")}>Settings</button>
         <button title="Emergency &amp; support resources" onclick={openHelp}>Emergency Resources</button>
       </nav>
@@ -1430,7 +1430,7 @@
     <!-- ============ BY SUBSTANCE ============ -->
     {#if tab === "bysub"}
       <section class="card">
-        <h2>By substance</h2>
+        <h2>Substance Log</h2>
         {#if usage.length}
           {#each usage as u}
             <div class="usage">