feat: add snippet library for prompt templates

- Add Rust backend with persistent storage for snippets
- Include 8 default snippets across categories (Code Review, Debugging, Testing, etc.)
- Create SnippetLibraryPanel component with category filtering
- Support create/edit/delete operations for custom snippets
- Default snippets can be edited but not deleted
- Add snippet button to InputBar for quick access

Closes #22

src/lib/components/InputBar.svelte

@@ -25,6 +25,7 @@
     type SlashCommand,
   } from "$lib/commands/slashCommands";
   import AttachmentPreview from "$lib/components/AttachmentPreview.svelte";
+  import SnippetLibraryPanel from "$lib/components/SnippetLibraryPanel.svelte";
   import type { Attachment } from "$lib/types/messages";
 
   const INPUT_HISTORY_KEY = "hikari-input-history";
@@ -39,6 +40,7 @@
   let selectedCommandIndex = $state(0);
   let attachments = $state<Attachment[]>([]);
   let isDragging = $state(false);
+  let showSnippetLibrary = $state(false);
 
   // Input history state
   let inputHistory = $state<string[]>([]);
@@ -617,6 +619,16 @@ User: ${formattedMessage}`;
     }
   }
 
+  function handleSnippetInsert(content: string): void {
+    // Insert snippet at cursor position or append to input
+    if (inputValue.trim()) {
+      inputValue = inputValue + "\n\n" + content;
+    } else {
+      inputValue = content;
+    }
+    userHasTyped = true;
+  }
+
   function handleKeyDown(event: KeyboardEvent) {
     // Handle command menu navigation
     if (showCommandMenu && matchingCommands.length > 0) {
@@ -723,6 +735,29 @@ User: ${formattedMessage}`;
     </div>
 
     <div class="button-wrapper">
+      <button
+        type="button"
+        onclick={() => (showSnippetLibrary = true)}
+        class="attach-button"
+        title="Snippet Library"
+      >
+        <svg
+          width="20"
+          height="20"
+          viewBox="0 0 24 24"
+          fill="none"
+          stroke="currentColor"
+          stroke-width="2"
+          stroke-linecap="round"
+          stroke-linejoin="round"
+        >
+          <path d="M14.5 2H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V7.5L14.5 2z" />
+          <polyline points="14,2 14,8 20,8" />
+          <line x1="16" y1="13" x2="8" y2="13" />
+          <line x1="16" y1="17" x2="8" y2="17" />
+          <line x1="10" y1="9" x2="8" y2="9" />
+        </svg>
+      </button>
       <button type="button" onclick={handleFilePicker} class="attach-button" title="Attach files">
         <svg
           width="20"
@@ -769,6 +804,13 @@ User: ${formattedMessage}`;
   </div>
 </form>
 
+{#if showSnippetLibrary}
+  <SnippetLibraryPanel
+    onClose={() => (showSnippetLibrary = false)}
+    onInsert={handleSnippetInsert}
+  />
+{/if}
+
 <style>
   .input-bar {
     display: flex;