fix: streaming scroll override, Gemini 3.x models, read-only workspace, two-container UID — v0.50.87 (closes #677 #669 #670 #668)

- #677: renderMessages() and appendThinking() use scrollIfPinned() during stream; scroll threshold 80→150px; floating ↓ scroll-to-bottom button added
- #669: Gemini 3.1 Pro Preview, 3 Flash Preview, 3.1 Flash Lite Preview added to all provider sections; gemini-3.1-flash-lite-preview was the missing ID causing API_KEY_INVALID; GEMINI_API_KEY env var detection added
- #670: docker_init.bash guards chown/write-test with [ -w ]; :ro workspace mounts no longer crash startup
- #668: UID/GID auto-detect probes /home/hermeswebui/.hermes and HERMES_HOME before /workspace; two-container Zeabur/Compose setups inherit correct UID automatically
- 18 new tests; 1441 total passing

static/style.css

@@ -428,6 +428,9 @@
   .workspace-toggle-btn:disabled{opacity:.38;cursor:not-allowed;}
   .chip.model{color:var(--accent-text);border-color:var(--accent-bg-strong);background:var(--accent-bg);}
   .messages{flex:1;overflow-y:auto;display:flex;flex-direction:column;min-height:0;position:relative;z-index:0;-webkit-overflow-scrolling:touch;touch-action:pan-y;overscroll-behavior-y:contain;}
+  /* sticky-first-child: button is first child of .messages so its natural position is above viewport; sticky+bottom:16px pins it there when visible */
+  .scroll-to-bottom-btn{position:sticky;bottom:16px;align-self:flex-end;margin-right:20px;width:32px;height:32px;border-radius:50%;border:1px solid var(--border2);background:var(--code-bg);color:var(--muted);font-size:16px;cursor:pointer;display:flex;align-items:center;justify-content:center;box-shadow:0 2px 8px rgba(0,0,0,.25);z-index:10;transition:color .12s,border-color .12s,background .12s;}
+  .scroll-to-bottom-btn:hover{color:var(--text);border-color:var(--border);background:var(--hover-bg);}
   .messages-inner{margin:0 auto;width:100%;padding:20px 24px 32px;display:flex;flex-direction:column;}
   @media(min-width:1400px){.messages-inner{max-width:1100px;}}
   @media(min-width:1800px){.messages-inner{max-width:1200px;}}