Add comment threading and fix fetch authentication

- Add comment threading/reply functionality with nested display
  - Database migration for parent_comment_id and thread_depth columns
  - Recursive comment rendering with depth-based indentation
  - Reply form with inline UI and smooth animations
  - Thread collapse/expand capability
  - Max thread depth of 3 levels

- Fix 401 authentication errors on API calls
  - Add credentials: 'same-origin' to all fetch calls
  - Affects settings.js, ticket.js, dashboard.js, advanced-search.js
  - Ensures session cookies are sent with requests

- Enhanced comment styling
  - Thread connector lines for visual hierarchy
  - Reply button on comments (up to depth 3)
  - Quote block styling for replies

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

migrations/002_add_comment_threading.sql

@@ -0,0 +1,19 @@
+-- Migration: Add comment threading support
+-- Adds parent_comment_id for reply/thread functionality
+
+-- Add parent_comment_id column for threaded comments
+ALTER TABLE ticket_comments
+ADD COLUMN parent_comment_id INT NULL DEFAULT NULL AFTER comment_id;
+
+-- Add foreign key constraint (self-referencing for thread hierarchy)
+ALTER TABLE ticket_comments
+ADD CONSTRAINT fk_parent_comment
+FOREIGN KEY (parent_comment_id) REFERENCES ticket_comments(comment_id)
+ON DELETE CASCADE;
+
+-- Add index for efficient thread retrieval
+CREATE INDEX idx_parent_comment ON ticket_comments(parent_comment_id);
+
+-- Add thread_depth column to track nesting level (prevents infinite recursion issues)
+ALTER TABLE ticket_comments
+ADD COLUMN thread_depth TINYINT UNSIGNED NOT NULL DEFAULT 0 AFTER parent_comment_id;