fix: global search case-insensitive + keyboard layout swap

Unified search normalization across InternalLinkPicker and GlobalSearch:

1. GlobalSearch.svelte: multi-variant search (same as InternalLinkPicker)
   - expandKeyboardVariants() for RU/EN layout swap
   - Parallel Search queries with dedup by type+nodeId+targetId+title
   - 180ms debounce preserved

2. Backend: fix LOWER() in SQL for links/actions
   - Replace LOWER(column) LIKE with lowercased columns (title_lower, url_lower, etc.)
   - Migration 020: add lowercased columns + indexes for links and actions
   - BackfillLinksLower() + BackfillActionsLower() in storage.go
   - Update INSERT in bindings_links.go and action.go to populate lowercased columns

3. FTS5 search: Unicode case-insensitive
   - Index lowercased title/content/tags in search_index
   - sanitizeFTS() now lowercases query before MATCH
   - RebuildFTS() called after migrations

4. Case-insensitive search for nodes (already done in previous commit, verified):
   - title_lower column with Go strings.ToLower
   - Search() queries title_lower with lowercased query

All test suites PASS, full build OK.

frontend/src/lib/GlobalSearch.svelte

@@ -1,6 +1,7 @@
 <script>
   import { createEventDispatcher, onDestroy, onMount, tick } from 'svelte'
   import { t } from './i18n'
+  import { expandKeyboardVariants } from './util/keyboardLayout'
 
   export let wailsCall = async () => []
   export let typeLabel = (type) => type || ''
@@ -42,9 +43,36 @@
     }
     loading = true
     try {
-      results = await wailsCall('Search', q) || []
+      // Build query variants: original + keyboard-swapped + lowercased
+      const variants = expandKeyboardVariants(q)
+      // Deduplicate preserving order
+      const seenVariants = new Set()
+      const queries = []
+      for (const v of variants) {
+        if (!seenVariants.has(v)) {
+          seenVariants.add(v)
+          queries.push(v)
+        }
+      }
+      // Execute all variant queries in parallel, collect results
+      const allResults = await Promise.all(
+        queries.map(v => wailsCall('Search', v).catch(() => []))
+      )
+      // Merge and deduplicate by node ID (or type+title for links/actions)
+      const merged = new Map()
+      // First pass: results from the original query get priority
+      for (let qi = 0; qi < allResults.length; qi++) {
+        const arr = allResults[qi] || []
+        for (const r of arr) {
+          const key = r.type + ':' + r.nodeId + ':' + (r.targetId || '') + ':' + r.title
+          if (!merged.has(key)) {
+            merged.set(key, r)
+          }
+        }
+      }
+      results = Array.from(merged.values())
       selectedIndex = 0
-      open = true
+      open = results.length > 0
     } catch (e) {
       results = []
       open = false