fix library sidebar: hide col: metadata tags, add sticky scroll

Two issues making year/sup/source tags at the bottom unusable:

1. col: namespace rendered 138 column-metadata buttons (62% of all
   tags) with 65-char labels, pushing useful tags below the fold.
   These are internal column documentation, not browsing categories.
   Now filtered out of the sidebar tag list.

2. Sidebar had no overflow scroll or sticky positioning. Tags below
   the viewport were unreachable. Now sticky with overflow-y: auto.

Sidebar now shows 85 tags (was 223), all reachable via scroll.

refs #71

docs/src/pages/library.tsx

@@ -155,6 +155,8 @@ export default function Library(): JSX.Element {
     const counts: Record<string, number> = {};
     for (const item of pool) {
       for (const t of item.tags) {
+        // Skip col: tags — column metadata, not useful for browsing
+        if (t.startsWith("col:")) continue;
         counts[t] = (counts[t] || 0) + 1;
       }
     }
@@ -202,7 +204,7 @@ export default function Library(): JSX.Element {
         }}
       >
         {/* Sidebar */}
-        <aside style={{ minWidth: 220, maxWidth: 260, flexShrink: 0 }}>
+        <aside style={{ minWidth: 220, maxWidth: 260, flexShrink: 0, maxHeight: "calc(100vh - 100px)", overflowY: "auto", position: "sticky", top: 80 }}>
           <h3>Collections</h3>
           <ul style={{ listStyle: "none", padding: 0, fontSize: "0.85rem" }}>
             <li>